The Session Initiation Protocol (SIP) is a communications protocol for signaling and controlling multimedia communication session such as voice and video calls. The most common applications of SIP are in Internet telephony, as well as instant messaging, over Internet Protocol (IP) networks.

The protocol defines the messages that are sent between endpoints, which govern establishment, termination and other essential elements of a call. SIP can be used for creating, modifying and terminating sessions consisting of one or several media streams. SIP is an application layer protocol designed to be independent of the underlying transport layer. It is a text-based protocol, incorporating many elements of the Hypertext Transfer Protocol (HTTP) and the Simple Mail Transfer Protocol (SMTP).[1]

SIP works in conjunction with several other application layer protocols that identify and carry the session media. Media identification and negotiation is achieved with the Session Description Protocol (SDP). For the transmission of media streams (voice, video) SIP typically employs the Real-time Transport Protocol (RTP) or Secure Real-time Transport Protocol (SRTP). For secure transmissions of SIP messages, the protocol may be encrypted with Transport Layer Security (TLS).
Some of the various reasons to use Sip are as follows

1) Pay only what you need - Charges are based on a per channel basis

2) Virtual Telphone Numbers - You may have phone numbers to other Areas of the country providing a local presend at a fraction of the cost (as little as $3 per month)

3) Still Provides Direct in Dial

4) Support for Direct to desk Faxing

5) Mobile call control and conferencing  - The ability to transfer calls from your cell phone and have multi party Conferences.

6) Eliminate the last mile saving monies.