Symptom
One-way audio issue faced wherein the agent on CUCM side cannot hear the PSTN side user if the call is transferred post ROC increment for SRTP call.
Call FLow: PSTN -- PRI -- ISR43XX -- MGCP -- CUCM -- sip -- CVP -- transfer to agent phone (after 20 minutes).
Flow of the call from the calling user perspective:
1. The external user makes a call which goes to the CUCM and through SIP signaling reaches the VVB (which is responsible for playing the welcome prompt and queue msg).
2. After some time (around 20 mins), the calling party will be transferred to the agent and at this point, the call gets transferred to the agent post the ROC rollover. What I mean by that is the ROC increments on the MGCP GW and the sequence number does reset to 0 while using the same SSRC and port.
3. At this point, once the call has been transferred to the agent, the agent cannot hear the PSTN user.
Conditions
1. Call flow involves MGCP GW.
2. SRTP is negotiated b/w the other leg (VVB/agent) and the MGCP GW.
3. The transfer of the call is made post the ROC increment. If the call is transferred before 21 minutes (for G711 negotiated),
Workaround
The only workaround identified at the moment is to change from SRTP to RTP.
