An Acronym for Session Initiation Protocol
A protocol for establishing a media session between two end-points. It doesn't carry the actual media, but can detect NAT, etc.
Some SIP services include:
Redirection: basically a way for a user on a dynamic IP address to be assigned a well-known name. You can register an account with a remote SIP server and when you are online, this server will tell incoming calls your real IP address.
Proxy: Your call is routed via the remote server.
