Question

Users at a branch site report that on some outbound PSTN calls through a centralized CUBE, the called party can hear the branch user, but the branch user hears nothing. Signaling completes normally and calls stay connected. A packet capture at the branch phone shows outbound RTP leaving the phone, but no inbound RTP arriving. The same phones work fine on internal calls. What is the MOST likely cause of this one-way audio?

Accepted Answer

An access control list or firewall rule along the return path is permitting signaling but blocking the inbound RTP UDP port range toward the branch subnet

Answer

The branch phones are configured with the wrong DSCP marking for media, causing the switch to drop EF-marked packets

Answer

The G.711 codec is mismatched, so CUBE cannot negotiate a common media stream in the SDP

Answer

The jitter buffer on the branch phones is undersized, discarding all inbound audio packets