NAT can interfere with SIP and RTP by changing the ports on the way through. To counter we enable NAT traversal by default therefore communicating directly to the port that sent us the original RTP traffic (instead of sending back to the RTP port in the SIP message). For SIP registrations therefore you should normally disregard any IP address and ports shown in the SIP message as in reality they are rarely the ones used.
Firewall Behind NAT
You can generally disable firewall rules if behind NAT as they they shouldn’t be required. However, if port forwarding [SIP and RTP] selecting an appropriate RTP range can be complicated. In this scenario you may need to disable NAT.
For incoming calls, if you’re users are reporting ringing with no voice on answer you’ve probably encountered a SIP ALG issue on your router. See our guide on Disabling SIP ALG on various brands of router.
If you are unable to access the router and therefore disable SIP ALG, consider switching your phone or PBX to TLS encryption (preventing SIP ALG) by changing the following two settings on your handset or IP-PBX.
- proxy: phone.tel2.co.uk
- port: 5061
- Transport: TLS