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What is SIP Trunking?

The definition of SIP trunking is a Voice over Internet Protocol (VoIP) based on session initiated protocol (SIP), which enables internet-based telephony vendors to provide telephone services and unified communications to customers with SIP-based private branch exchange (IP-PBX).

Why Use SIP Trunking?

A SIP trunk provides service that is equivalent to what you get from an analog phone line. However, the big difference is that a SIP trunk, connects your PBX to your network via a “virtual” wire using either your T1, cable modem, ethernet, etc.

One key benefit of using a SIP trunk is that you don’t need to maintain the same degree of physical infrastructure. SIP trunks are also appealing to large companies because it enables enterprises to pay for fewer phone lines. For companies that are using PBX systems, SIP trunking is often a far more attractive alternative to purchasing a separate phone line for every individual employee. And for fast-growing companies, SIP trunking helps make it easier to scale.

Another added benefit to using SIP trunks is that it is easier to connect to services that are hosted in the cloud. This can provide sales teams with the advantage of connecting PBX phones to sources of rich data like CRM, as well as other cloud-based tools in the sales technology stack.

SIP Trunking Applications

SIP trunking can enable a variety of applications that can provide unified communications services including:

  • Video
  • Voice
  • Streaming media
  • Web conferencing
  • Desktop sharing

SIP Trunking Architecture

SIP trunking architecture can partition a unified communications network into two disparate domains including:

  • A Private Domain: VoIP solutions that occur at a customer’s business or home
  • Public Domain: a full VoIP access solution to the PSTN / PLMN property and responsibility of the ITSP that provides phone service

These two domains must be interconnected through a SIP trunk. The interconnection is created by transport via the Internet Protocol (IP), which involves establishing specific regulations in order to handle some of the protocols and services that are classified as SIP trunking.


The ITSP is must follow law obligations of the Public Domain including:

  • Identification of users
  • Tracking traffic
  • Implementation of lawful interception mechanisms

The private domain is not subject to particular constraints of law, and can instead be either the responsibility of the ITSP, end users, or of the voice services provider.