NAT Traversal

Your router hides every device behind a single public address, which works beautifully for browsing: your computer asks for a web page and the answer comes straight back. Phone calls are different. Audio arrives as an incoming stream from a machine your phone never spoke to, and the router, seeing traffic nobody asked for, has no idea where to send it.

Three standard techniques solve this, and your phone system uses them without you configuring anything. STUN lets a phone ask an outside server what its public address looks like, so it can advertise the right one. TURN provides a relay to bounce audio through when nothing else works — reliable, but it adds a hop. ICE tries the available paths and keeps whichever succeeds. Most business phone services also hold the connection open by sending small keep-alive packets, so the router does not forget the phone between calls.

You will never buy NAT traversal or see it on an invoice, but you will absolutely notice it failing. One-way audio, calls that ring and then go silent, phones that stop receiving calls after a quiet night: these are the classic signs. They get reported as "the phone system is broken" when the real fault is in the building's router.

The practical value of knowing the term is that it tells you where to look. Instead of waiting on hold with your provider, you check the three things that break traversal locally: SIP ALG rewriting your call signaling, double NAT stacking two routers, and an aggressive firewall closing the return path too quickly. Providers ask about all three, and having the answers ready turns a multi-day ticket into a short call.