Question

A caller on an IP phone reports that when dialing an external number through the CUBE-connected PSTN provider, they hear silence instead of ringback tone until the far end answers. Using 'debug ccsip messages' on CUBE, you observe that the ITSP sends a SIP 183 Session Progress with SDP shortly after the INVITE, but the calling phone never plays local ringback. What is the MOST likely cause of the missing audio during the alerting phase?

Accepted Answer

The ITSP is providing in-band early media in the 183 Session Progress, but RTP from the provider is not reaching the phone, so no announcement or ringback is heard

Answer

CUCM requires a 180 Ringing response to generate ringback, and the 183 should be suppressed at CUBE so the phone plays local tone

Answer

The codec negotiated in the 183 SDP is G.722, which IP phones cannot decode during early media

Answer

A SIP 183 message is invalid without a prior 100 Trying, so the phone discards the response and stays silent