I am facing some issue during the SIP handshake in between voice gateway and CUCM which showing the error message of "503 Service Unavailable" while I had found that the reason/cause code for this error message was "Reason: Q.850;cause=47".
From my research, I found that the cause code was related to codec mismatch which I had added/enabled the MTP to the media resource group in call manager and by right it will have sufficient resources to encode/decode the codec. The call route was from voice gateway -> Cisco CUCM -> Cisco UCCX (IVR) -> agents but it will dropped after the resource unavailable in Cisco CUCM to encode/decode the codec before route to Cisco UCCX.
In region, it was configured with codec G.729 as preference from the SIP trunk and route to Cisco UCCX with the codec G.711 & G.722 as preference which is default for Cisco UCCX. I had try my best to fine tuning & troubleshoot on this issue for few days but unfortunately the problem was not resolved.
Kindly need some assistance & advice on the configuration for CUCM to resolve this issue.