Troubleshooting one way audio calls
- SIP ALG interference – this is related to the NAT scenario, where the SIP UA sits behind a router doing NAT and SIP ALG (Application Layer Gateway). Normally the SIP ALG should help the UA to traverse the NAT, but in most of the cases (due to its limited understanding over the SIP protocol) it ends up doing chances over the SDP that are not correct. Even if the ALG will change the private IPs in the SDP with the public IP of the NAT/router, it fails in properly replacing the SDP port correctly or maintaining the NAT pinhole open. The best way to deal with this is to actually try to disable the SIP ALG support in the router;
- bad RTP relay insertion – some SIP proxy on the path tried to insert a media relay (like rtproxy on rtprelay) in the media path, but it did in in the wrong way, like maybe not symmetric (on both directions), or the relying sessions was not properly set, etc.
- firewall presence – even if the actual SDP info is correct, sometimes a firewall may prevent a device from receiving the incoming RTP, so this is something worthy to check.