A SIP Trunk is a service; telephone call is routed over the IP backbone of a carrier using VoIP technology. SIP Trunks are used in conjunction with an IP-PBX and are thought of as replacements for traditional PRI or analog circuits. The popularity of SIP Trunks is due primarily to the cost savings of SIP, along with the increased reliability as backed by the SLAs of SIP Trunk Providers.

A SIP trunk is a service offered by an ITSP (Internet Telephony Service Provider) that permits businesses that have a PBX installed to use Voice-over-IP (VoIP) also outside the enterprise network by using the same connection as the Internet connection.

SIP trunking is a way to enjoy significant savings on your current phone bill. Using an Internet connection right from your current PBX, a SIP trunk uses SIP (Session Initiation Protocol) for a VoIP connection.

In these tougher economic times, more and more businesses, both small and large at looking at the advantages of SIP trunking and creating call treatments in their PBXs to take more chargeable calls out through these cost saving routes. And this trend is likely to substantially increase over years.

SIP trunking takes data, voice and video out over your Internet connection.

Some of the benefits of SIP trunking include:
• Much lower rates on long distance calls, International calls and in-bound toll free calls.
• The ability to establish virtual numbers in other geographic areas so that callers can use these numbers instead on more costly 800 numbers.
• Using virtual numbers from other areas to establish a “point of presence” for the company.
• Reduced costs as no TDM cards are required.
• Getting the benefits of Hosted VoIP without having to abandon existing equipment or investing in IP phones.
• The ability to easily add more calling trunks without the need for expensive digital PRI / analog station (port) cards.
• The ability to use an IAD for connecting regular analog phones, instead of having to buy more expensive equipment.
• Expansion of lines is dependent on bandwidth, which can easily be increased if needed. Additional lines can be set for compressed codecs (G729 and others) to use less bandwidth per call.

Ref: TMCnet.com