How the sides agree on audio
The caller sends an SDP offer: the address and port for voice, codecs in order of preference, DTMF and the direction. The answer picks codecs from that list and gives its own address. With no common codec, port 0 or the wrong direction, the call connects with no audio or one-way audio.
Common causes of silence
- A private address (192.168…, 10…) in the c= line: the phone is behind NAT and the PBX did not account for it.
- sendonly or inactive left over after a transfer or a hold nobody released.
- No common codec: the phone does only G.711 and the trunk only G.729.
- One side encrypts (RTP/SAVP), the other does not.
Related: SIP log analyzer, NAT test, codec bandwidth.