Yesterday I was cutting over a SIP Gateway from a 2010 pool to a newly built 2013 pool. Migrating the gateways and Trunk configuration is really simply.

What surprised me though is that once I had made the required config changes and published the Topology I was unable to make or recieve calls.

Looking at the SBC Gateway I found the error SIP 503 Service Unavailable for each attempted call setup. OK..

I fired up wireshark on the Mediation Server and found that the call was being sent to the Mediation Server but was being ignored …??? What’s going on here?

Checked connectivity and ports, all good.

Lets look at an outbound call to see if that reveals anything. 
An attempt to make the call fails immediately. This usually suggests that the call routing isn’t even engaging. Often a symptom of call routing issues. 

Since I never changed any call routing I  just confirmed that the Gateway is correctly associated with the route I was using, all good.

Another culprit is often that the gateway is marked as down. A quick look at the Event Log confirmed by the Event ID 46046 stating that the gateway has been marked as down. Once the gateway is down Lync won’t even try to send the call to the gateway hence the immediate failure.

Usually restarting the Lync Mediation Service will reset the communication to the Gateway. But in my case this didn’t happen. The gateway stays “down”.

After much head scratching I noticed that the Mediation service wasn’t able to resolve the Lync Edge Pool name. Since its recommended that the Edge Servers are not domain joined their DNS records need to be manually added.  

Once the Mediation Server was able to resolve the Edge Pool Name the calls started working.

Go figure…