SIP 503 after Migrating Gateway to Lync 2013 pool

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…


About Paul B

My name is Paul Bloem and I am employed at Lexel Systems in New Zealand as a Principal Consultant for Unified Communications. I have been working on enterprise voice solutions for over 20 years. My first 10 years were spent working for a Telco in South Africa (Telcom SA). This is where all the groundwork happened as I was exposed to just about every aspect of telecommunication you could imagine. I develop an interest in PBX technologies and eventually became the go-to guy. Next, I had a 10 year run at Siemens South Africa, most of my time there was as a Technical Trainer. During this time VoIP hit the world stage, I had the privilege of introducing VoIP both as H.323 and later SIP across the Siemens HiPath 4000 solution stack. In 2008 I immigrated to New Zealand with my newly attained MCSE, I was ready to go where no PBX Techie had gone before. I was employed to explore OCS 2007 and that was pretty much the beginning of the end for me. I have been working on OCS and Lync ever since. My current role focuses exclusively on Lync and associated technologies.. That includes pre-sales, consulting, architecture and design, training and support. I even get to play in the development space from time to time - focus on play ;-) I was nominated as a Microsoft VTSP for Lync early in 2013 and also awarded Microsoft's MVP award for Lync in 2014.
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