Session Initiation Protocol (SIP) is a communications protocol that is widely used for managing multimedia communication sessions such as voice and video calls.  SIP, therefore is one of the specific protocols that enable VoIP.   It defines the messages that are sent between endpoints and it governs establishment, termination and other essential elements of a call. SIP can be used to transmit information between just two endpoints or many.  In addition to voice, SIP can be used for video conferencing, instant messaging, media distribution and other applications.  SIP has been developed and standardized under the auspices of the Internet Engineering Task Force (IETF).

When most people hear the term VoIP, they imagine a hosted VoIP solution where the vendor hosts and operates the PBX functionality that manages call handling, voicemail and other applications.  The customer’s IP enabled phones connect to the Internet and ultimately to the vendor’s servers and software.  This, however is just one type of VoIP deployment.