CUCM - Incoming calls via SIP trunk get connected to wrong gateway

[TomServo6]

Hello,

We have a strange issue where intermittently (about 30-40% of the time) calls inbound from another PBX (that come in via a SIP trunk) are getting the wrong gateway from Call Manager so the calls do not connect correctly. Packet capture show this, for example:

Working Call Flow

Outside PBX > SIP TRUNK > CUCM > Phone

Failed call flow (Call connects, but no way audio)

Outside PBX > SIP TRUNK > CUCM > Incorrect H323 Gateway > (UDP stream cannot get to phone due to network routing)

The gateway for working calls is showing as CUCM. The voice gateway for non-working calls is showing one of our H323 gateways, which is incorrect so the call gets no-way audio. Again this is intermittently but I can't find why CUCM is sending the calls to our H323 gateway in this case. 

CUCM Version is 8.6, we have 4 servers in the cluster. Any help is appreciated.

[nathanjgrace]

If you use dialed number analyzer on the trunk, then type the digits the other PBX would send, do you see any alternate matches? I am wondering if there's a wildcard pattern that might get selected which would send the calls to the other gateway. 

Do you have any CAC, AAR, or regions/bandwidth configured?

When a successful call comes in through the trunk, does the trunk dial the DN of the phone directly or does the CSS point it to a translation pattern? 

[TomServo6]

Thanks for the response, this issue ended up being related to MTP resources, the reason for the intermittent problem was CUCM was running out then trying to use the gateway which was causing the issue.