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.

One of the major uses of SIP is to assign a user a well-known name, sort of like an email address, that they can use no matter where they connect from.

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. It also means you can take your hardware VoIP phone with you and plug it in anywhere (assuming you get assigned a routable IP address).

Proxy: Your call is routed via the remote server.

If you are having troubles configuring your SIP phone. Try using IP addresses for everything, instead of domain names. SIP phones sometimes try to use SRV records to configure itself, however if it's an IP address it'll just do it all itself.

Compare to XMPP.