SIP is the open standard used for the traffic of voice on IP (VoIP or
Voice Over IP). Contrary to Skype which uses a protocol owner, SIP
makes it possible to inter-connect various systems between them thanks
to the opening of the protocol. SIP becomes the standard impossible to
circumvent of telecommunications. A great number of software as the
private telephone switch (PABX) Asterisk (software free) support
standard SIP.