There are times where you check the RTP streams in your VoIP capture and everything looks sane, although one or more parties in the call reported no audio.
Those times, in particular when G.711 is in use, can be made a little less frustrating by looking at the RTP payload.
In fact, it's possible that one party is just sending silence. With G.711 silence has its own coding, and it's easy to spot:
a-law: silence is either a payload entirely populated with 0x55 or 0xD5 (depending on the sign applied).
u-law: silence is a payload entirely populated with 0xFF.