The setup
A five-port analogue gateway (ATA) registering directly to a carrier SIP trunk, with no SBC in the path. Each port needed its own phone number.
The original design had one port holding the real credentials and the other four relying on a routing-ID field to tell inbound calls apart. It worked intermittently, then regressed to routing every inbound call to the same port, whatever number was dialled.
The root cause
Syslog and SIP traces showed it: the SIP User ID was set to the trunk’s shared auth username instead of each port’s own number.
The fix
Register each port independently, with its own number as the SIP User ID, sharing one auth credential. It worked immediately, with no SBC or outbound proxy.
Before calling it production-ready
- Intermittent transport errors pointed to SIP ALG on the router, and to allowing the carrier’s full IP range.
- A short registration expiry times five ports means a lot more signalling, so re-verify each port before go-live.
- Confirm the carrier is happy with multiple endpoints on one credential.