SIP Trunks Explained

What is a SIP trunk?

SIP trunk, IP and VoIP.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 explained.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.