SIP Trunking

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.

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. This would, therefore, give you 20 unused channels. With SIP, it is possible for you to add, subtract or move a line by simply making arrangements with your provider. This saves you cost by never requiring you to have more or less lines than you actually need. Even if you should have to move your office, with SIP, you are able to keep all the same internal extensions and inbound numbers. Basically, SIP Trunks cater to your needs by allowing you to purchase in additions of one instead of overloading your office with costly, unnecessary additions.

Easy Implementation and Integration with Existing System

With the use of an Integrated Access Device (IAD), the SIP-ready PBX required can be easily modified to function with key or customary analog systems. The necessity of PRIs, BRIs, or local PSTN is completely eliminated with SIP as I mentioned above.

As systems become more complex, it is recommended that you increase the speed and capacity of your Internet. If you move E1 and T1 Interfaces to two, you will need additional hardware.

Multi-site businesses may not communicate via multiple sub PRI connections. Instead of the multiple sub PRI connections, they may communicate via a single SIP Trunking account.
To build an infrastructure will require you to purchase an IP-PBX, IP enabled devices, IP Phones, or softphones (software telephones that make it possible to place voice calls through your computer and over the Internet) all of which are relatively inexpensive.

Another wise step you should take is to put a firewall that is SIP-aware in place in order to regulate your VOIP traffic. The return on investment (ROI) of these systems is normally both obvious and fast, and the switch will be well worth the minimal cost and time. You certainly won’t regret making the switch.

Emergencies that may cause the disabling of your communications at your location are easily overcome:

  • Via predetermined failover services, your SIP trunks will automatically redirect calls to additional locations and extensions. For instance, should a power outage occur, your calls will be automatically redirected to a satellite office.
  • It is also possible for many businesses to set forwarding options to their smartphones since the use of mobile devices and the practice of bring your own device (BYOD) have grown so rapidly in today’s business world. Mobile devices can be a good backup if your primary communications go down. In addition, since a number of these systems are optimized for networks that require low bandwidth use of data or Wi-Fi, the calls most likely will be totally free.

SIP Offers Flexibility

If you’re looking for outsourcing or your company has an international presence, SIP can meet your needs. For example, SIP can spread your customer service staff across the country or even around the world. Therefore, SIP makes it possible for your business to be open 24 hours a day and 7 days a week.

There is no need for a phone line at all with SIP. A Session Initiation Protocol Uniform Resource Identifier (SIP URI) address includes numbers and the name of the host. This system entirely bypasses the PSTN and has no cost at all to connect to a different SIP number. Many workers whether on-site or remote prefer to use their own IP-Enabled devices for SIP. These IP-Enabled devices include PDAs, tablets, smartphones and laptops.

Highlights of SIP

SIP offers small businesses the opportunity to appear both large and professional, and large businesses benefit by maintaining a local presence. Whether your business is small or large, the most significant feature of any system is for it to be reliable and able to add to your bottom line. VOIP business services and specifically SIP will do that for you.

The pricing, services and features of SIP Trunking vary greatly with the different providers.

To be added...

  • PSTN-to-PSTN calls traversing SIP networks

See Also:

Business SIP Providers
Provider Plan Details Monthly Rate *
OnSIP OnSIP
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8x8 8x8 IP Trunking
  • Unlimited calls to US and Canada
  • Softphone and mobile app available
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Broadvoice SIP BroadVoice SIP
  • 5 lines for $55, additional lines only $2
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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.

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. This would, therefore, give you 20 unused channels. With SIP, it is possible for you to add, subtract or move a line by simply making arrangements with your provider. This saves you cost by never requiring you to have more or less lines than you actually need. Even if you should have to move your office, with SIP, you are able to keep all the same internal extensions and inbound numbers. Basically, SIP Trunks cater to your needs by allowing you to purchase in additions of one instead of overloading your office with costly, unnecessary additions.

Easy Implementation and Integration with Existing System

With the use of an Integrated Access Device (IAD), the SIP-ready PBX required can be easily modified to function with key or customary analog systems. The necessity of PRIs, BRIs, or local PSTN is completely eliminated with SIP as I mentioned above.

As systems become more complex, it is recommended that you increase the speed and capacity of your Internet. If you move E1 and T1 Interfaces to two, you will need additional hardware.

Multi-site businesses may not communicate via multiple sub PRI connections. Instead of the multiple sub PRI connections, they may communicate via a single SIP Trunking account.
To build an infrastructure will require you to purchase an IP-PBX, IP enabled devices, IP Phones, or softphones (software telephones that make it possible to place voice calls through your computer and over the Internet) all of which are relatively inexpensive.

Another wise step you should take is to put a firewall that is SIP-aware in place in order to regulate your VOIP traffic. The return on investment (ROI) of these systems is normally both obvious and fast, and the switch will be well worth the minimal cost and time. You certainly won’t regret making the switch.

Emergencies that may cause the disabling of your communications at your location are easily overcome:

  • Via predetermined failover services, your SIP trunks will automatically redirect calls to additional locations and extensions. For instance, should a power outage occur, your calls will be automatically redirected to a satellite office.
  • It is also possible for many businesses to set forwarding options to their smartphones since the use of mobile devices and the practice of bring your own device (BYOD) have grown so rapidly in today’s business world. Mobile devices can be a good backup if your primary communications go down. In addition, since a number of these systems are optimized for networks that require low bandwidth use of data or Wi-Fi, the calls most likely will be totally free.

SIP Offers Flexibility

If you’re looking for outsourcing or your company has an international presence, SIP can meet your needs. For example, SIP can spread your customer service staff across the country or even around the world. Therefore, SIP makes it possible for your business to be open 24 hours a day and 7 days a week.

There is no need for a phone line at all with SIP. A Session Initiation Protocol Uniform Resource Identifier (SIP URI) address includes numbers and the name of the host. This system entirely bypasses the PSTN and has no cost at all to connect to a different SIP number. Many workers whether on-site or remote prefer to use their own IP-Enabled devices for SIP. These IP-Enabled devices include PDAs, tablets, smartphones and laptops.

Highlights of SIP

SIP offers small businesses the opportunity to appear both large and professional, and large businesses benefit by maintaining a local presence. Whether your business is small or large, the most significant feature of any system is for it to be reliable and able to add to your bottom line. VOIP business services and specifically SIP will do that for you.

The pricing, services and features of SIP Trunking vary greatly with the different providers.

To be added...

  • PSTN-to-PSTN calls traversing SIP networks

See Also:

Created by: SCR, Last modification: Mon 18 of May, 2015 (00:24 UTC) by admin
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