Start with direction
Identify whether audio is missing both ways or only toward one endpoint. One-way audio almost always narrows the fault to one advertised media address, firewall direction or NAT rewrite.
Read the negotiated path
A codec mismatch matters only after packets reach both sides. Zero packet counters point to routing or firewall policy first.
- Compare SDP c= addresses and m= audio ports on both call legs.
- Check local and remote media addresses in the call detail.
- Compare inbound and outbound RTP packet counters before changing codecs.
- Disable ICE for ordinary SIP phones unless the endpoint and PBX path are designed for it.
Prove the fix
Place a new call and retain its call-session ID. Confirm RTP in both directions and use the optional PCAP module only when packet-level evidence is still required.