Well, danramos.. yes and no. It CAN use TCP and it CAN use UDP. Citing: http://en.wikipedia.org/wiki/Session...ation_Protocol "SIP works in concert with several other protocols and is only involved in the signaling portion of a communication session. SIP clients typically use TCP or UDP on port numbers 5060 and/or 5061 to connect to SIP servers and other SIP endpoints." So SIP could operate independent of the TCP/IP framing--like any well behaved networking application should. So yes--you COULD be using a SIP client that uses UDP and it could conceivably be designed to jump carriers and interfaces if the application layer is written so be so fault tolerant, since it would be the responsibility of the combined effort of the network and application layers to handle such an even. I hope that answers your question, Dan.