What is SIP Trunking?

Updated: May 21, 2021



SIP trunking can revolutionize your business by letting businesses take a giant leap to upgrade their PBX to an internet-powered phone system.

It’s more reliable and lets you optimize all your business communications. From crisp voice calls to high-quality video, you’ll never risk your reputation with a poor calling experience.

Running your phone system through a SIP trunking is affordable, but it's not necessarily for everyone. It replaces the need for physical lines. It also lets you easily add new phone lines or modify your phone service without disruptions or the need to buy new hardware.

Everyone from small businesses to large enterprises can benefit from SIP trunking. If you want to see why and how, this is the guide for you.

In this post, we're going to cover what SIP Trunking is and why you should consider it for your business:

  • What is SIP Trunking?

  • What is SIP Trunking Used For?

  • How Does it Work? SIP Trunking Explained

  • What is a Direct Inward Dialing Number (DID)?

  • What Are the Benefits of SIP Trunking?

  • SIP Trunking vs. PRI: Which One is Right For You?

  • What is the Difference Between SIP and VoIP?

  • How to Get Started With a SIP Trunking Service?

What is SIP Trunking?

SIP, short for Session Initiation Protocol, is an application layer protocol that lets you run your phone system over an internet connection instead of traditional phone lines. Trunking refers to the backbone of phone lines used by multiple users that connects to a telephone network.

SIP Trunks provision Voice over Internet Protocol (VoIP) connectivity between an on-premise phone system and the public switched telephone network, also known as PSTN. For instance, an office might have an existing PBX for phone service. SIP trunks provide phone service for the entire office so they can reach the outside world. It operates similar to HTTP, which is the very basis of browsing web pages on the internet. SIP was embraced by the open-source community and acted as a catalyst for commercial phone service. Also, SIP's capabilities extend beyond phone service, into broader messaging functions.

Just like you’d get a traditional phone line subscription from a landline provider, SIP trunking is provided by a SIP provider.

An excellent SIP trunking service lets you use your existing PBX equipment to transition into an internet-powered telephone network.

And that’s not all: you can also use a SIP phone as part of your unified communications system. You can synchronize all your communication channels in real-time. This way, you'll enhance productivity, collaboration, and efficiency across your company.'

What is SIP Trunking Used For?

The main role of SIP trunking is to replace PRI technology. PRI, or Primary Rate Interface, has been used for decades to deliver lines of voice and data using physical copper lines. Essentially, it's a bundle of analog phone lines put together. PRI can be costly to maintain since the hardware is becoming outdated rather quickly. With SIP trunking, this happens using VoIP technology. SIP also supports other data like instant messages and video conferencing.

SIP trunking is the right choice for anyone ready to move their business communication on a light budget.

This warrants a follow-up question: how will you know that your company is ready for SIP? Here are some questions to consider:

  • Are you currently paying for call capacity that you don’t need?

  • Is it expensive to maintain all your equipment?

  • Do your voice calls drop unexpectedly or need higher reliability overall?

  • Do you have remote employees and/or employees that travel or relocate often?

In short, SIP trunking is used for call setup, management, and teardown. It’s flexible, reliable, and quite lightweight. This means you can focus on your business instead of the technicalities. SIP is the basis for VoIP communications and SIP Trunking is used to provide VoIP connectivity through a PBX.


How Does SIP Trunking Work? SIP is the dominant format used in IP telephony. It’s a protocol that establishes a VoIP session over the internet. In this session, you can have any of the following scenarios and more:

  • Be on a voice call with one other person

  • Have multiple people on a conference call

  • Run video calls

SIP trunks work as an intermediary between your business phone system and the Internet Telephony Service Provider (ITSP). See the diagram below showing how calls flow from a SIP phone (VoIP) in a business to the outside world—it's fascinating! In the past, companies used ISDN circuits that needed to be physically installed on their premises. Now, instead of routing voice calls over copper lines in these circuits, SIP trunking send