In several enterprise projects I have seen in one direction RTP streams with empty RTP payload, i.e. $55 values only from the beginning instead of real audio. This happens a few times a week, while hundreds of calls show bidirectional audio. To me there seems to be no systematical schema - incoming as well as outgoing calls (call initiation) are affected - the empty RTP streams sometimes are directed from the enterprise customer to the carrier - sometimes in the reverse direction - apparently never in both directions. Furthermore it doesn't matter, what kind of external user is involved (mobile phone or wireline users). As far as the SIP signalling is concerned I (hopefully) have checked everything thats relevant (port# for media traffic are OK, senrecv parameter is OK). Such calls usually last between 20 .. 40 seconds, as the other party ('hallo ?') will give up soon and terminate the call.
Would be great, if anyone can give me a hint ... thx in advance !
Software/Hardware used: VoIP (SIP) calls, RTP protocol