Very useful in determining causes
Source http://msdn.microsoft.com/en-us/library/gg132446(v=office.12).aspx

Mode	Error ID	Error Message	Additional Information	Diagnostic reason
IM	0	This message was not delivered to [user name] because the service is not available.		Server Internal Error.
IM	1	[user name] could not be reached and this message was not delivered.		Service Unavailable.
Conference	4	The network is busy. Cannot join the meeting.		Insufficient bandwidth to establish session. Do not attempt re-route.
Conference	4	Cannot add [user name] to the meeting because network is busy.		Insufficient bandwidth to establish session. Do not attempt re-route.
Voice	4	Could not complete the call because the network is busy. Try again later.		Insufficient bandwidth to establish session. Do not attempt re-route.
Voice	4	Cannot complete the transfer.	Transfer failed because the network is busy. Please try again later.	Insufficient bandwidth to establish session. Do not attempt re-route.
Voice	4	Could not add video because the network is busy. Try again later.		Insufficient bandwidth to establish session. Do not attempt re-route.
Voice	5	This call may take longer to connect because the network is busy. Some functionality may be reduced.	This call may take longer to connect because the network is busy. Some functionality may be reduced.	Insufficient bandwidth to establish session. Attempt PSTN re-route.
Conference	1003	This online meeting cannot be found.	This online meeting cannot be found on the server. Please check the meeting ID and try again.	User does not exist.
Conference	1003	[user name] cannot be found. Please check the address and try again.		User does not exist.
IM	1003	[user name] could not be found and this message was not delivered:		User does not exist.
Voice	1003	Cannot locate [user name].	There may be an error in the address. Check the address and then try again.	User does not exist.
Voice	1003	Cannot complete the transfer.	This phone number or address is incorrect, or it is outside of your organization and is not federated with your company. Please contact your support team with this information.	User does not exist.
Conference	1004	[user name] did not respond to the invitation.		Route set is no longer valid.
Sharing	1004	Sharing ended due to network issues. Try again later.		Route set is no longer valid.
Conference	1005	Cannot join this meeting from outside your organization.	Cannot join the meeting because the meeting organizer is outside of your organization and is not federated with your company. Or, the meeting information may be incorrect. Please contact your support team with this information.	Cannot route to destination domain.
Conference	1005	Cannot reach [user name].	The address is incorrect or this person is outside your company and not federated. Please contact your support team with this information.	Cannot route to destination domain.
IM	1005	This message was not delivered to [user name] because the address is outside of your organization and is not federated with your company, or the address is incorrect. Please contact your support team with this information.		Cannot route to destination domain.
Sharing	1005	[user name] did not receive your sharing invitation.	[user name] did not receive your sharing invitation because the address is incorrect or this person is outside your company and not federated. Please contact your support team with this information.	Cannot route to destination domain.
Sharing	1005	[user name] did not receive your sharing invitation.	[user name] did not receive your sharing invitation because the address is incorrect or this person is outside your company and not federated. Please contact your support team with this information.	Cannot route to destination domain.
Voice	1005	Cannot locate [user name].	This phone number or address is incorrect, or it is outside of your organization and is not federated with your company. Please contact your support team with this information.	Cannot route to destination domain.
Sharing	1007	Screen sharing cannot connect to the server now.	Screen sharing cannot connect to the server now. Please wait and try again. If this issue continues, contact your support team with this information.	Temporarily cannot route.
Conference	1008	Cannot join this meeting from outside your organization.	Cannot join the meeting because the meeting organizer is outside of your organization and is not federated with your company. Or, the meeting information may be incorrect. Please contact your support team with this information.	Unable to resolve DNS SRV record.
Conference	1008	Cannot reach [user name].	The address is incorrect or this person is outside your company and not federated. Please contact your support team with this information.	Unable to resolve DNS SRV record.
IM	1008	This message was not delivered to [user name] because the address is outside of your organization and is not federated with your company, or the address is incorrect. Please contact your support team with this information.		Unable to resolve DNS SRV record.
Sharing	1008	[user name] did not receive your sharing invitation.	[user name] did not receive your sharing invitation because the address is incorrect or this person is outside your company and not federated. Please contact your support team with this information.	Unable to resolve DNS SRV record.
Voice	1008	Cannot locate [user name].	This phone number or address is incorrect, or it is outside of your organization and is not federated with your company. Please contact your support team with this information.	Unable to resolve DNS SRV record.
Voice	1008	Cannot complete the transfer.	This phone number or address is incorrect, or it is outside of your organization and is not federated with your company. Please contact your support team with this information.	Unable to resolve DNS SRV record.
Conference	1009	Cannot join this meeting from outside your organization.	Cannot join the meeting because the meeting organizer is outside of your organization and is not federated with your company. Or the meeting information may be incorrect. Please contact your support team with this information.	No match for domain in DNS SRV results.
Conference	1009	Cannot reach [user name].	The address is incorrect or this person is outside your company and not federated. Please contact your support team with this information.	No match for domain in DNS SRV results.
IM	1009	This message was not delivered to [user name] because the address is outside of your organization and is not federated with your company, or the address is incorrect. Please contact your support team with this information.		No match for domain in DNS SRV results.
Sharing	1009	[user name] did not receive your sharing invitation.	[user name] did not receive your sharing invitation because the address is incorrect or this person is outside your company and not federated. Please contact your support team with this information.	No match for domain in DNS SRV results.
Sharing	1009	[user name] did not receive your sharing invitation.	[user name] did not receive your sharing invitation because the address is incorrect or this person is outside your company and not federated. Please contact your support team with this information.	No match for domain in DNS SRV results.
Voice	1009	Cannot locate [user name].	This phone number or address is incorrect, or it is outside of your organization and is not federated with your company. Please contact your support team with this information.	No match for domain in DNS SRV results.
Voice	1009	Cannot complete the transfer.	This phone number or address is incorrect, or it is outside of your organization and is not federated with your company. Please contact your support team with this information.	No match for domain in DNS SRV results.
Voice	1011	The call could not be completed. Please try again later.		Ms-diagnostics header not provided by previous hop.
Conference	3026	Cannot join the meeting as an anonymous attendee.	Cannot join the meeting because unauthenticated users are not allowed. Contact the meeting leader for more information.	Conference does not allow anonymous users.
Conference	3027	You do not have permission to join this meeting.	Cannot join the meeting because you do not have permission. Please contact the meeting leader for more information.	User not allowed in closed conference.
Conference	3028	Cannot join the meeting as an anonymous attendee.	Cannot join the meeting. Joining as an unauthenticated user is not supported. Contact your support team for more information.	Anonymous access disabled globally.
Conference	3032	This online meeting cannot be found.	This online meeting cannot be found on the server. Please check the meeting ID and try again.	Conference does not exist.
Conference	3033	The conferencing service did not respond. Please wait and try again.		The C3P transaction timed out.
Sharing	3033	The conferencing service did not respond. Please wait and try again.		The C3P transaction timed out.
Conference	3040	The meeting you are trying to join has ended.	The meeting you are trying to join has ended or does not exist. Contact your support team with this information.	Conference is not active.
Conference	3041	Cannot join the meeting due to user permissions.	The information provided to the conferencing service was incorrect, or you may not have permission to join the meeting. Contact your support team with this information.	Unknown participant.
Conference	3043	Cannot join the meeting.	The meeting is not configured to allow people to join. Contact the meeting organizer for more information.	The organizer policy is not configured or is invalid.
Conference	3044	Cannot join the meeting as an anonymous attendee.	Cannot join the meeting because unauthenticated users are not allowed. Contact the meeting leader for more information.	Anonymous access is not allowed.
Conference	3097	The conferencing service did not respond. Please wait and try again.		No MCU is available via the MCU factory.
IM	3097	[user name] could not be reached and this message was not delivered:		No MCU is available via the MCU factory.
Sharing	3097	Screen sharing cannot connect to the server now.	Screen sharing cannot connect to the server now. Please wait and try again. If this issue continues, contact your support team with this information.	No MCU is available via the MCU factory.
Sharing	3097	Screen sharing cannot connect to the server now.	Screen sharing cannot connect to the server now. Please wait and try again. If this issue continues, contact your support team with this information.	No MCU is available via the MCU factory.
Voice	3097	Call was unsuccessful.	The conferencing service is not available. Please wait and try again. If this problem continues, contact your support team.	No MCU is available via the MCU factory.
Conference	3098	The conferencing service did not respond. Please wait and try again.		No MCU Factory Available.
Conference	3099	Cannot connect to this type of online meeting.	This meeting is not configured to allow people to join. Contact the meeting organizer for more information.	The requested media is not configured for this conference.
Conference	3100	Meeting is full.	Cannot connect to the meeting because it is full. Contact the meeting leader for more information.	Maximum participant count for this meeting has been exceeded.
Conference	3100	Cannot invite [user name] because the meeting is full.		Maximum participant count for this meeting has been exceeded.
Sharing	3108	Screen sharing cannot connect to the server now.	Screen sharing cannot connect to the server now. Please wait and try again. If this issue continues, contact your support team with this information.	MCU is unreachable.
Sharing	3109	Screen sharing cannot connect to the server now.	Screen sharing cannot connect to the server now. Please wait and try again. If this issue continues, contact your support team with this information.	The request was sent to the MCU but the C3P transaction timed out.
Sharing	3110	Sharing is disabled.		The MCU rejected addConference request. This usually indicates a misconfiguration.
Conference	3113	A presenter has ended the meeting.		Conference Terminated – Deleted.
Conference	3114	The meeting ended because the hosts have left.		Conference Terminated – Enterprise User Absent.
Conference	3115	This meeting was ended due to inactivity.		Conference Terminated – Inactivity.
Conference	3116	A presenter has ended the meeting.		Conference Terminated – Organizer Ended Session.
Sharing	3116	A presenter has ended the meeting.		Conference Terminated – Organizer Ended Session.
Conference	3118	You were removed from the meeting.	A presenter has removed you from the meeting. For more information, please contact the meeting organizer or a presenter.	Participant Removed.
Sharing	3118	You were removed from the meeting.	A presenter has removed you from the meeting. For more information, please contact the meeting organizer or a presenter.	Participant Removed.
Conference	3119	You were not admitted into the meeting.		Participant Denied.
Conference	3120	Call was disconnected.	The call was disconnected because the audio ended. This can happen if there are delays in the network. Try logging out of	Participant session expired.
IM	3122	This message may not have been delivered to [user name] because there was no response from the server.		The C3P request sent to the MCU failed.
Sharing	3122	Screen sharing cannot connect to the server now.	Screen sharing cannot connect to the server now. Please wait and try again. If this issue continues, contact your support team with this information.	The C3P request sent to the MCU failed.
Conference	3138	The subject line is too long.	The online meeting cannot be created because the subject is too long. Please shorten the subject and try again.	The requested conference subject exceeds the maximum length.
Conference	3181	Meeting permissions are preventing you from joining.	The permissions set for this meeting require you to end your audio call before you can join by using the meeting link. Please hang up and then click the meeting link again. Once you have joined the meeting, you may call to join audio again if you need to.	Rich join is not allowed for lobby bypassed user.
Voice	4002	Cannot complete the call.	There is more than one contact with your phone number. If you cannot resolve this problem, contact your support team with this information.	Multiple users associated with the source phone number.
Voice	4004	The provided credentials are not authorized by the server.		Credentials provided are not authorized to act as specified from URI.
Voice	4009	Cannot contact [user name].	There is more than one contact with the same phone number. Try calling again. If you still cannot complete the call, contact your support team with this information.	Multiple users associated with the destination phone number.
Conference	5001	The conferencing service did not respond. Please wait and try again.		Request timed out.
Conference	5001	Cannot reach [user name]. Try again later.		Request timed out.
IM	5001	This message may not have been delivered to [user name] because there was no response from the server.		Request timed out.
Voice	5001	Call was unsuccessful.	The server timed out. Wait and try again. If the problem continues, contact your support team with this information.	Request timed out.
Conference	5026	The conferencing service did not respond. Please wait and try again.		Request was sent to Callee Client but it timed out.
Conference	5026	Cannot reach [user name]. Try again later.		Request was sent to Callee Client but it timed out.
IM	5026	This message may not have been delivered to [user name] because there was no response from the server.		Request was sent to Callee Client but it timed out.
Voice	5026	Call was unsuccessful.	The server timed out. Wait and try again. If the problem continues, contact your support team with this information.	Request was sent to Callee Client but it timed out.
IM	6000	[user name] could not be reached and this message was not delivered:		SIP request send failed.
Conference	6001	[user name] did not respond to the invitation.		Request time-out.
IM	6001	[user name] could not be reached and this message was not delivered:		Request time-out.
IM	6002	[user name] could not be reached and this message was not delivered:		Dialog/Transaction does not exist.
IM	6004	The message was not delivered. [user name] is signed in to a device that cannot receive messages.		Media type is not supported.
IM	6005	This message was not delivered to [user name] because the service is not available:		Invite session is not set up.
IM	6006	This message was not delivered to [user name] because the service is not available:		MCU is shutting down.
IM	6007	This message was not delivered to [user name] because the service is not available:		Conference URI not present in request.
IM	6008	This message was not delivered to [user name] because the service is not available:		Conference represented by conf uri does not exist.
IM	6009	This message was not delivered to [user name] because it is too large:		Message has exceeded maximum allowed length.
IM	6010	You are not authorized to send messages to [user name]. This message was not delivered:		Authorization failure.
IM	6011	[user name] could not be found and this message was not delivered:		User not found.
IM	6012	[user name] could not be found and this message was not delivered:		Participant is not expected to be incoming.
IM	6013	This message was not delivered to [user name] because the service is not available:		Conference is terminated.
IM	6014	This message was not delivered to [user name] because the service is not available:		Unexpected invite or reinvite request.
IM	6015	This message was not delivered to [user name] because this type of content could not be received:		SDP is not acceptable.
IM	6017	This message was not delivered to [user name] because it is too large:		INFO body has exceeded maximum allowed length.
IM	6024	[user name] cannot receive messages in the format you used. This message was not delivered:		Message does not contain a format acceptable to the client.
Conference	7012	Cannot start the online meeting due to encryption issues.	The conferencing service does not support the type of content encryption you are using. Contact your support team for more information.	Cannot support media encryption required by client.
Voice	7012	Cannot start the online meeting due to encryption issues.	The conferencing service does not support the type of content encryption you are using. Contact your support team for more information.	Cannot support media encryption required by client.
Voice	7013	Cannot connect to the online meeting.	The conferencing requires content encryption to join the meeting. Contact your support team with this information.	SRTP-encrypted media required to join.
Voice	10006	Cannot connect the call due to a server error. Please try again		Proxy-side media negotiation failed.
Voice	10009	The server is not responding or cannot be reached. Please try your call again later.		Mediation server is shutting down.
Voice	10400	The call cannot be completed as dialed.	The call cannot be completed as dialed. Verify the dialing rules for your organization and try again.	Gateway responded with 400 Bad Request.
Voice	10503	The server cannot forward the call. Please try again.		Gateway responded with 503 Service Unavailable.
Voice	12000	The call could not be completed. Please try again later.	The call could not be completed because of a busy network. Please try again later.	Routes available for this request but no available gateway at this point.
Voice	12001	Your organization does not support calling this number.		User policy does not contain phone route usage.
Conference	12013	Conference does not permit unauthenticated users to join from or dial out to another device.		Conference dialouts from anonymous users disallowed.
Voice	12013	Conference does not permit unauthenticated users to join from or dial out to another device.		Conference dialouts from anonymous users disallowed.
Conference	13002	[user name] cannot be reached and may be offline.		The routing rules did not result in a final response.
Voice	13002	[user name] is unavailable or may be offline.		The routing rules did not result in a final response.
Voice	13002	Cannot complete the transfer.	Call was not transferred because [user name] cannot be reached and may be offline.	The routing rules did not result in a final response.
Conference	13010	The network is busy. Cannot join the meeting.		Target rejected call because of bandwidth congestion.
Conference	13010	Cannot add [user name] to the meeting because network is busy.		Target rejected call because of bandwidth congestion.
Voice	13010	Could not complete the call because the network is busy. Try again later.		Target rejected call because of bandwidth congestion.
Voice	13010	Cannot complete the transfer.	Could not complete the call because the network is busy. Try again later.	Target rejected call because of bandwidth congestion.
Voice	14005	Unable to place call. Please check the number and try again.		Could not find profile in internal lookup.
Conference	14010	Call was not completed or has ended.	Cannot complete the call. To correct this problem you may need to provide an area code, a number to access an outside phone line, or a number to dial long distance. If you cannot resolve this problem, your system administrator may need to change the geographic location on your user account or update the company dialing rules.	Unable to find an exact match in the rules set.
Conference	14010	Cannot reach [user name].	The address is incorrect or this person is outside your company and not federated. Please contact your support team with this information.	Unable to find an exact match in the rules set.
Voice	14010	Please check the number and try again.	The format of the number you dialed is not valid. Examples of valid dialing formats are as follows: 1+(555) 555-0123 (555) 555-0123 9+1+(555) 555-0123 011+ 12 34 56 78 99 If you continue to be unable to successfully make a call, please contact your support team.	Unable to find an exact match in the rules set.
IM	16002	This message was not delivered to [user name] because the message contents are restricted by policy. For more information, contact your system administrator.		Policy disallows message contents.
Sharing	21002	Attendees not allowed to share in this conference.		Attendees cannot share in this conference.
Sharing	21003	Sharing failed to connect due to network issues. Try again later.	Sharing failed to connect because of network issues. Try again later.	The media connection with the client was lost.
Sharing	21004	This sharing has ended because another type of sharing has started.		A presenter has started sharing content.
Sharing	21005	Application sharing ended because you joined from another client.		This user has joined the conference from another endpoint.
Sharing	21006	Application sharing failed because media failed to set up correctly.		The RDP connection failed to establish.
Sharing	21007	Application sharing ended because the application sharing conference has ended.		User removed because of conference ending.
Sharing	21008	Application sharing ended because the server is shutting down.		The Application Sharing Conferencing Service is shutting down.
Sharing	21012	Sharing failed to connect due to network issues. Try again later.	Sharing failed to connect because of network issues. Try again later.	The media connection with the client was lost.
Voice	25001	This call may take longer to connect because the network is busy. Some functionality may be reduced.	This call may take longer to connect because the network is busy. Some functionality may be reduced.	Retrying via PSTN because of bandwidth congestion.
Voice	26014	Cannot complete the call because you are no longer an agent for [user name].		User not authorized to make outbound call on behalf of the Response Group.
Voice	26019	Service is temporarily unavailable.		Response Group settings found but is not active.
Voice	35000	No call is parked at this number.	You have dialed a number used for retrieving parked calls. No call is currently parked at this number. The call may have been retrieved or it may have ended. If you were not trying to retrieve a parked call or know that the parked call is still parked, please check the number and try again.	Orbit not found.
Voice	35002	Cannot park call right now. Please try again later.		No orbits are available.
Voice	35007	Cannot park call right now. Please try again later.		Park request failed. Cannot establish a connection between the Call Park application and the parkee.
Voice	35015	The call was ended while it was on hold.		Park time expired. Terminating parkee call leg.