What is a SIP trunk?

SIP trunks are connections that link a business IP-PBX to the PSTN (Public Switched Telephone Network). Through an (ISP) Internet VoIP connection signaling goes from the business phone system to the SIP (Session Internet Protocol) provider as a virtual phone line, who then connects and establishes voice calls. The trunk connects to the PTSN. SIP trunks are much more cost effective than traditional types of phone services. They can lower phone costs using "unlimited" plans, as well as pay by the minute plans with lower bulk minute rates. Some providers offer exceptionally low rates for particular regions or countries.

SIP trunks are virtual phone lines which can be used to place and receive phone calls over the Internet to anyone in the world with a valid phone number.

SIP trunk service is significantly less expensive than traditional business telephone lines. Sometimes as much as 60 to 75% lower.

The number of SIP channels needed by a business will depend on the type of business and their call activity. Additionally, it will depend on the provider and how they construct their pricing. Typically each SIP trunk can transmit a number of SIP channels. Each channel is capable of transmitting one call. SIP providers typically offer customers options to subscribe to a specified number of SIP channels.
Since one SIP trunk is capable of an unlimited number of channels, you would merely require one SIP trunk with a specific provider. However, incorporating several SIP trunks, each from a different provider offers increased benefits by setting up cost effective call rules.

Using a VoIP PBX appliance with SIP Trunks

Configuration of SIP trunks can be done by tech savvy people and providers often offer guidance.

A small office with three people would require one trunk per person if all three were on the phone at the same time. As an office size increases the ratio of trunks per person would likewise decrease to about sixty percent. Call centers and high call volume businesses would need a high ratio of trunks or active virtual phone lines to working employees.

Pros of Using SIP trunks:

Lower call costs: Select SIP providers with low costs per minute or by a region or country.
Low cost unlimited plans: Select a SIP provider with one low monthly cost for an unlimited number of minutes. (Many of these plans include the lower 48 US states and portions of Canada.)
Scalability: Increasing or adding trunks is quick and easy.
Multiple Carriers: SIP trunks can be set up for different SIP Providers from the same IP-PBX for both cost efficiencies and failover.
Disaster Recovery: Using SIP trunks calls can be rerouted instantly.
Shared Resources: SIP trunks use the businesses' Internet connection which dynamically allocates bandwidth to active voice calls, otherwise it's used for other apps.
Less Maintenance: There is no need for PSTN (or PRI) hardware, copper phone lines or circuit boxes.

Diagram showing company using VoIP PBX with SIP trunks.

Advantages of a VoIP PBX appliance using SIP Trunking Services

Call Anywhere for the Lowest Costs

For businesses that generate lots of International calls SIP providers can supply unlimited calling plans to many countries or incredibly low rates. The most cost effective approach to International calling is by incorporating SIP trunks. Some providers offer incredibly cost effective services to certain parts of the world. For instance a pharmaceutical supply company that does significant business with firms in India might pick a well established Indian SIP provider. SIP trunking empowers companies to gain access to these providers. Using SIP trunks between multiple offices on the same VoIP system become free Peer to Peer calls.

The Greatest Flexibility and Control

SIP trunks offer the most flexibility of any phone services. The precise nature of SIP allows for multiple trunks from numerous providers to be added or subtracted instantaneously. The instant flexibility provides concise control by scaling trunks as needed using the most cost effective providers.

Instant Failover and Disaster Recovery

Regular analog trunks (PSTN phone lines) can go down from bad weather or inferior circuits. These instances routinely require a few hours minimum to correct, especially for PRI circuits. SIP trunks offer immediate virtual failover lines where calls can instantaneously be routed. Once setup these Outbound Call Rules configured in the VoIP appliance do not require administration or time consuming maintenance.

SIP trunking vs Hosted VoIP

The choice between a Cloud based business phone service (hosted VoIP) and an on-premise IP-PBX using SIP trunks often takes into account the size of the company and the quantity of calls. Typically most small companies or offices with up to about 20 employees would most likely be better with a hosted VoIP solution. However, as a companies' number of employees and/or needed trunks (lines) increase an on-premise IP-PBX with SIP trunks offers both cost efficiencies and a larger level of flexibility. A second factor would be International calling and the lower costs associated with some SIP trunk providers. Companies using SIP trunks can employ multiple SIP service providers choosing routes based on specific rates and time of day. Additionally, having complete control of the IP-PBX and using preferred codecs and SD-WAN software, a company can increase the number of simultaneous calls on lower bandwidth pipes. This flexibility can be important for some companies.

An additional critical consideration for installing an on premise VoIP PBX would be having personnel capable of administering the appliance. Most SIP providers do offer easy set up instructions for a number of PBXs. They also often provide phone support with assistance for configuration or troubleshooting technical issues.

Office to Office Free Calling

Incorporating SIP trunks between multiple offices on the same VoIP system become free Peer to Peer calls. You can set up extension dialing between office, regardless of where they are. Connecting multiple sales offices worldwide can save thousands of dollars.

If you currently have your own PBX in-house phone system, then it may have SIP trunking capabilities. Setting up a SIP trunk connection can be a worthwhile endeavor. By opening up a lower cost phone service you can reduce your phone call expenses.

Growth of SIP Trunk Forecast

The annual growth rate predictions for Global SIP trunk services are currently just under 19% compounded annually.

Bandwidth Requirements

For a small office a good rule of thumb is to calculate about 90 Kbps for each active call for both the download and upload for a high quality uncompressed G.711 codec. Using compression to save bandwidth the G.729 codec would require about 32 Kpbs.

A more technical answer would be for G.711, which many high quality VoIP providers use, requires 64 Kbps of bandwidth and G.729 8 Kpbs. However, these amounts do not include IP (Internet Protocol) overhead (The headers that contain data with important information.), which adds an additional 23 Kpbs.

SIP Trunks Explained