Question

A SIP trunk connects CUCM to a third-party voicemail gateway. Outbound calls from CUCM to the gateway fail. In a packet capture, CUCM sends an INVITE, the gateway responds with '401 Unauthorized' containing a WWW-Authenticate header, and CUCM immediately sends a CANCEL followed by re-attempting the identical INVITE with no Authorization header. The loop repeats until the call fails. What is the most likely cause?

Accepted Answer

The SIP trunk's outbound INVITE requires digest authentication, but CUCM's SIP trunk security profile has no configured digest credentials to respond to the challenge

Answer

The gateway and CUCM have mismatched supported codecs, causing the 401 rejection

Answer

CUCM is sending the INVITE over UDP while the gateway only accepts TCP transport

Answer

The gateway's TLS certificate is not trusted by CUCM, forcing repeated authentication failures