Getting the settings right on the Cisco Unified Communications Manager (CUCM)side of a SIP trunk is sometimes a fine art and there are certainly many things that can go wrong. This week I ran across two errors that at first glance were not completely obvious as to what the issue was.
503 Service Unavailable
Service unavailable is a misleading error to say the least. Just when you think that you have everything setup right things don’t work as they should. Well at least this time around it half worked. Inbound calls to Lync from CUCM seemed to work just fine but outbound calls to CUCM failed. As the screen shot below shows the Device Pool is set just like the documentation shows with the Device Pool set to default.
Well this is all good in a lab that has one CUCM server but in a production environment that has multiple CUCM subscribers the default Device Pool may not be setup with the correct Communications Manager Group (CMG). In the case I am describing the default Device Pool had the incorrect CMG defined and even though calls from CUCM to Lync worked fine, calls from Lync to CUCM failed. They generated a 503 Service Unavailable error message from CUCM when looking at Snooper Traces on the Lync Mediation Server.
So the lesson learned is always ensure you have the correct CMG defined in your device pool you associate to your SIP trunk. The Servers defined in the CMG are the same CUCM servers/gateways that are defined in your Lync Topology.
404 Not Found
This error although a little simpler to understand doesn’t mean that the solution is all that easy to find. Basically a 404 means either the line number doesn’t exist, there is not a matching route pattern or there are some Calling Search Space (CSS) issues with your CUCM configuration.
Probably the most complex is Calling Search Space (CSS) issues. Although the CUCM Dialed Number Analyzer may seem as though things should work chances are CSS issue may still be the issue. Best bet is to create Partitions and Calling Search Spaces just for the inbound calls setting on your Lync SIP Trunk in CUCM so you understand exactly what is occurring with calling permissions.
Hopefully this sheds some light on a couple of common errors I have seen in CUCM Lync Direct SIP deployments.