Troubleshooting One-Way Audio on VoIP Calls

One-way audio means one party on a call can hear the other, but not vice versa. This is almost always a NAT (Network Address Translation) issue — the audio (RTP) stream from one direction cannot find its way back through the firewall/NAT. Common causes: SIP ALG (Application Layer Gateway) on the router is interfering with SIP packets — disable SIP ALG in the router settings (it does more harm than good with modern VoIP). The VoIP device's external IP is not correctly reported to the SIP server — configure the device's external IP or STUN server address. Fix on the PBX: in 3CX or similar, configure the external IP/FQDN and the RTP port range. Open the RTP port range (commonly 10000–20000 UDP) on the firewall — both inbound and outbound. On the VoIP provider side, ensure STUN/TURN settings are configured. If the issue only occurs with calls to/from specific destinations, the problem may be on the remote carrier's network.