From the SIP RFC 4904:
A Session Initiation Protocol (SIP) to PSTN gateway may have trunks that are connected to different carriers. It is entirely reasonable for a SIP proxy to choose — based on factors not enumerated in this document — which carrier a call is sent to when it proxies a session setup request to the gateway. Since multiple carriers can transport a call to a particular phone number, the phone number itself is not sufficient to identify the carrier at the gateway. An additional piece of information in the form of a trunk group can be used to further pare down the choices at the gateway. As used in this document, trunks are necessarily tied to gateways, and a proxy that uses trunk groups during routing of the request to a particular gateway knows and controls which gateway the call will be routed to, and knows what trunking resources are present on that gateway.
In an architecture where calls can be terminated on multiple gateways it is wise to consider routing the call to a destination based on some significant criteria such as cost, quality or proximity. Where a proxy has the ability to evaluate a call based on one or more of these criteria, as well as knowledge of the TDM trunk resources available, the proxy can "tag" the call using the tgrp and trunk-context values in the SIP Contact field of the INVITE. It is important to note that the tgrp and trunk-context values can only be used with a TEL URI, not with a SIP URI.
Unlike in traditional telephony, where bundles of physical wires were once delivered from the service provider to a business, a SIP trunk allows a company to replace these traditional fixed Public Switched Telephony Network (PSTN) lines with PSTN connectivity via a SIP trunking service.
Your voice, data and videos are all combined into a single line with Session Initiation Protocol (SIP) Trunking. This allows for your local, long distance and broadband Internet service to be combined into one line. You will be able to keep your real time traffic off the internet as well as off the public switched telephone network (PSTN) as much as conceivable.
Another factor that can be costly for your business is if you want to up the number of Primary Rate Interfaces (PRI) from 23 to 24 channels, you must buy a second PRI that has 24 channels. ...
A Session Initiation Protocol (SIP) to PSTN gateway may have trunks that are connected to different carriers. It is entirely reasonable for a SIP proxy to choose — based on factors not enumerated in this document — which carrier a call is sent to when it proxies a session setup request to the gateway. Since multiple carriers can transport a call to a particular phone number, the phone number itself is not sufficient to identify the carrier at the gateway. An additional piece of information in the form of a trunk group can be used to further pare down the choices at the gateway. As used in this document, trunks are necessarily tied to gateways, and a proxy that uses trunk groups during routing of the request to a particular gateway knows and controls which gateway the call will be routed to, and knows what trunking resources are present on that gateway.
In an architecture where calls can be terminated on multiple gateways it is wise to consider routing the call to a destination based on some significant criteria such as cost, quality or proximity. Where a proxy has the ability to evaluate a call based on one or more of these criteria, as well as knowledge of the TDM trunk resources available, the proxy can "tag" the call using the tgrp and trunk-context values in the SIP Contact field of the INVITE. It is important to note that the tgrp and trunk-context values can only be used with a TEL URI, not with a SIP URI.
Unlike in traditional telephony, where bundles of physical wires were once delivered from the service provider to a business, a SIP trunk allows a company to replace these traditional fixed Public Switched Telephony Network (PSTN) lines with PSTN connectivity via a SIP trunking service.
What is SIP Trunking and how will it help my business?
Basically, SIP Trunking is a service that provides VOIP or Voice over Internet Protocol. In other words, it is a form of communicating by transmitting telephone calls over the Internet. This communication through the Internet is done by connecting the private branch exchange (PBX) to the Internet. The Internet actually replaces the telephone trunk allowing for communication by users with both fixed and mobile telephone subscribers throughout the world.Your voice, data and videos are all combined into a single line with Session Initiation Protocol (SIP) Trunking. This allows for your local, long distance and broadband Internet service to be combined into one line. You will be able to keep your real time traffic off the internet as well as off the public switched telephone network (PSTN) as much as conceivable.
Advantages of SIP Trunking
The advantages of SIP Trunking over traditional telephone lines and older VOIP protocols are several:- Whereas before SIP you needed to carry voice, video and data over one line by using a Primary Rate Interface (PRI), the SIP Trunking eradicates the necessity for gateways of Basic Rate Interfaces (BRI), Primary Rate Interfaces (PRIs) and PSTN.
- The provisions of incoming, outgoing and Private Branch Exchange (PBX) are made by your VOIP business provider setting up a proxy server also known as a SIP proxy.
- Your provider also does all technical support. This saves you both time and money since you will no longer need an IT team or an IT contractor.
SIP Trunking Saves You Money
SIP Trunking allots lower costs without sacrificing quality. When it comes to pricing, SIP Trunks are significantly cheaper than the customary analog circuits. What is the saving? The cost of SIP trunks will range from approximately $20 to $30 per trunk, whereas the analog circuits cost roughly $30 for each circuit. There are also significant savings with charges of long distance terminations with SIP Trunks costing considerably less than TDM rates or customary analog rates. All calls are local with SIP. The result for your customers is that both incoming and outgoing calls have an area code that is local. This gives you a lower cost for your business, and your customers get a feeling of familiarity and closeness with your business. The cost of SIP calls per minute are only a fraction of a penny. In addition, SIP numbers that are toll-free are also available to you.Another factor that can be costly for your business is if you want to up the number of Primary Rate Interfaces (PRI) from 23 to 24 channels, you must buy a second PRI that has 24 channels. ...