What happens in SIP that is so sensitive to address and port translations?
Two things that SIP does can be messed up by NAT: First, your IP phone has registered itself with the SIP Registrar Server operated by your VoIP Service Provider because this allows your VoIP Service Provider to route inbound calls to your IP phone. (For an explanation of the registration process, click here). However, if the IP address given to the Registrar server is the private LAN address of the IP phone, not the address on the external interface of your NAT router, then the Service Provider may not be able to send SIP messages to your IP phone. Second, when a SIP phone makes a call it sends a SIP INVITE request. Within that request, it also sends details of how it should be reached for the audio media stream as described above. The connection used for the SIP messages that start and end calls is not the same connection as is used to send the audio stream. The audio stream is always established on a new connection using a completely different port number. NAT routers normally allow
