SIP stands for Session Initiation Protocol. It is an application layer protocol.

SIP is for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, instant messaging, multimedia conferences, and multimedia distribution.
Please only submit Voice over IP websites that specifically, or primarily, relate to the SIP protocol.
SIP based VoIP and IM Service Providers.
SIP only service providers in here please. Sites already listed should be @ linked rather than re-listed.