One-way audio on SIP calls behind NAT
by voip.kb lablast verified 2026-06-30tested 2026-05-12
Problem
Inbound SIP calls connect but only one party hears audio when the PBX sits behind a NAT router.
Symptoms
- Caller hears nothing
- Callee hears audio
- Call setup succeeds
Root cause
RTP media is advertised with the private IP in SDP because external IP settings are unset.
Verification evidence
Reproduced on three FreePBX 16 installs; packet captures confirm RTP sent to RFC1918 address.