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SIP Trunk

What is SIP Trunk?

Generally speaking, people in the industry associate "SIP Trunk" with the place where you can connect a SIP call to a PSTN (analog) line. When a call is directed to SIP Trunk, one would expect the call to be connected (or terminated) from an IP network to a PSTN line.

In terminology sense, SIP Trunk can provide phone lines for your IP telephony network just like your home telephone line from AT&T or some other local telephone carrier. The fact that IP lines are not physical lines, like the ones in PSTN networks, means that SIP Trunk providers are providing virtual "lines" to people.

There are many ITSPs (Internet Telephony Service Providers) that mainly provide termination services from SIP to PSTN lines. Since services like Vonage or Google Voice support SIP, they can be categorized as SIP trunk service providers.

What does SIP Trunk service do?

  1. Provide connectivity with analog lines (SIP <-> PSTN or PSTN <-> SIP)
    Since not all voice networks support SIP, SIP Trunk usually provides termination to PSTN lines. Usually SIP Trunk offers competitive toll charges for long distance or international calls compared to traditional telephone carriers.

  2. Connect SIP clients to other SIP clients (SIP <-> SIP)
    As the name indicates, SIP trunk supports SIP (Session Initiation Protocol). Thus, any SIP devices and services can connect directly to the network without any devices or gateways in between. Some SIP Trunk services even offer free connections from SIP to SIP devices/services. I am not aware of any SIP Trunk service provider that offers only SIP-SIP communication since most of them usually offer SIP <-> PSTN as their main service.

  3. Handle any real-time communications based on SIP
    Not only does SIP Trunk cover the basics of communication, but it can also provide additional means of communication as well. Any SIP services or applications that support SIP can run through SIP Trunks. These communication methods include Instant Messaging (IM), Presence, Video, etc.

Who needs SIP Trunk service?

If some of the descriptions below fit your situation, you may want to look into SIP Trunk services:
  • Multiple offices scattered among various states/countries
  • Need more than two lines at a time
  • Need a "virtual" presence at remote locations (e.g., a New York local number for customers in NY, even though your company may be located in a different city, such as San Francisco)
  • Want to minimize toll charges for long distance or international calls
  • Make many SIP calls

Benefits of SIP Trunk

  • Add lines as you go (not married to or limited to a specific number of lines)
  • Fast deployment
  • Easier configuration and low maintenance/training
  • Reduced hardware cost
  • Usually offer competitive toll charges for international and long distance calls
  • Local number portability
  • May offer more choices in disaster recovery

Things to consider before signing up

  • Interoperability
    Make sure your IP-PBX supports SIP standards. (Some IP-PBXs say they support SIP, but they actually support a proprietary version of "SIP" instead.) If you have analog PBX, you may need to install a SIP-analog gateway between your PBX and SIP trunk.

  • Transitioning
    Since it is easy to increase the number of lines with SIP Trunk providers, it may be smart to start with just a few so that you can test the environment before committing all of your lines of communication to a SIP Trunk service account.
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