There can be some confusion on the topic of SIP or SIP trunks.  If you ask four people what a SIP trunk is you may get four different answers.

 “It’s an internet line”, “A call that runs over VoIP”, “It’s a replacement for PRI”, or “It allows for free calling.”

SIP GrapgThese are all some of the typical responses. According to SSCA® Sip Training, a basic description of SIP trunking is, “Voice communications using IP based connectivity to supplement or replace traditional telephony service circuits (DS0, DS1, PRI), allowing an organizations telephone system to connect to the PSTN.”

SIP trunks are not trunks in the old telephony sense in that they are not hard wired circuits.  Instead, they use IP connections to originate or terminate telephone calls to and from the Public Switched Telephone Network.  Businesses and companies have been connecting their PBX’s to the PSTN by TDM based trunks for decades.  This is usually a traditional T1 or PRI connection containing 23 or more voice trunks.  SIP trunking is a virtual connection alternative to a service provider who then can pass calls onto the PSTN.  SIP enabled PBX’s connect to an IP based link, such as a DSL line.


A couple of the main benefits of SIP trunks are:

  • Lower cost – Typically 30% to 50% cheaper per trunk when utilizing a converged broadband service.  
  • Greater functionality – with no hard direct connection to deal with, the trunks can be moved easily to other locations / systems.  For example, in a disaster recovery situation.

There can be a few challenges with SIP including Quality of Service (QoS) and security. It is important to employ QoS techniques, or configurations in routers and switches that prioritize voice traffic over data traffic.  Including separating the DATA and Voice traffic on the local customer network. It is also important that the broadband line is symmetric, meaning the connection has the same upload and download speeds as both are equally utilized for voice traffic.

The popularity of SIP trunking is increasing as enterprises look for the flexibility, cost savings, and scalability that comes with the service. According to a recent Infonetics report, over ¾ of enterprises surveyed plan on using SIP trunks in the next year.

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