You seem to be confused about SIP. SIP is the "Session Initiation Protocol", and its entire point is to coordinate initiation of a direct session between endpo

You seem to be confused about SIP. SIP is the "Session Initiation Protocol", and its entire point is to coordinate initiation of a direct session between endpoints, the so-called "SIP Trapezoid". SIP is a lot like SMTP. When you make a SIP call, you talk to your local SIP server, which talks to N intermediaries (usually 0) before going out to the public internet. After passing off to the destination server, the connection can go through N intermediaries on the remote side before being delivered to a final user agent. This connection isn't for delivering audio and video, though, it's for coordinating a direct RTP connection to use for audio and video. Once the two endpoints are communicating, they send information about their real network location, work out details about NAT, and set up a direct RTP connection, possibly using STUN to deal with NAT issues. In a normal SIP environment, neither party's SIP server ever sees any of the actual media. This is all fairly similar to what skype does, except skype has additional infrastructure for routing media through other clients, in cases where two endpoints can't find a way to communicate directly, as I understand it. I'm not actually authoritatively-informed about skype, though, so don't rely on this last bit.