VoIP glossary · Network Address Translation
What is NAT?
NAT (Network Address Translation) is the router function that lets many devices with private IP addresses share one public address, and it is the most common cause of one-way audio on SIP calls.
NAT explained
SIP puts IP addresses inside its messages, in the Contact and Via headers and in the SDP. Behind NAT, a PBX may advertise its private address (such as 10.0.0.5), which the carrier cannot reach. Signalling might still work, but audio sent to that private address goes nowhere.
The fix is to tell the PBX its public address and local networks so it rewrites headers correctly, open the RTP port range on the firewall, and turn off SIP ALG on the router. Every setup guide on this site includes NAT notes for that platform.
Example