What is SIP Trunking: Your Questions Answered

Technology | 4 minute read

The on-going contact center conversion to SIP trunking technology has been an industry success story. Why? For starters, using SIP technology to enable voice and video calls, messaging, and data transmission saves money on telco costs: up to 50% more, according to Gartner analysts.

And SIP trunking is super-flexible; it makes it easy for contact centers to add advanced features like speech analytics, Web Call-Backs (formerly Visual IVR), and Scheduled Call-Backs from third-party vendors. It’s the right solution for many, but there are questions to be answered before you know whether SIP trunking is right for your contact center.

What is SIP Trunking?

Before we go any further, it’s important to understand what a trunk — and, more specifically, a SIP trunk — is. In telecom, a trunk is a set of connections that serves multiple users and carries many phone calls at a time. Every call center uses trunks to carry its calls.

Think of a SIP trunk as the online version of a phone line. SIP, or session initiation protocol, is a protocol that sets up communications through voice, video, and messaging applications. A SIP-enabled trunk lets you use an internet connection instead of a traditional phone line to connect callers together.


The “trunk” in SIP trunk is a legacy term left over from the days of analog calling, when a trunk line was an actual physical bundle of phone lines connected to businesses. Because a SIP trunk delivers the same functionality, the name stuck.

What are the benefits of SIP trunking?

SIP trunking is popular for good reason. It comes with many benefits, one of the main ones being that it allows you to use the same IP-based network for multiple types of data transmission, including voice and video calls. Some other benefits include:

  • SIP trunking frees you from having different subscription services for voice and data.
  • It can lower your hardware costs, as hundreds of VoIP calls can be made using the same hardware footprint.
  • SIP trunking lets you build a single, solid infrastructure that scales with your business and supports your growth without requiring you to constantly add new services along the way.
  • Can be up to 50% cheaper than traditional telco setups and
  • Can easily facilitate advanced features like Web Call-Backs (formerly Visual IVR) and speech analytics.

The flexibility of SIP is one of its biggest advantages. Because it is a standardized protocol that can enable voice, video, and messaging data, external vendors can interact with your call center platform without requiring any custom development. This frees the contact center from being stuck with the onboard options that come with their contact center software and lets them integrate the specialized software, like Voice Call-Backs, that best suits their needs.

Is SIP the same as VoIP?

SIP and VoIP, or voice over internet protocol, make a great team, but they are not the same thing. Instead, SIP is one of the most widely used protocols to implement VoIP. VoIP itself is a suite of technologies and protocols that enables voice data to be sent over the internet, while SIP works more in the background, and is used to set up and end VoIP calls.


SIP can be used to enable communications in a variety of media, not just voice.


The more traditional way of carrying phone lines is the public switched telephone network, or PSTN. While both have their own advantages, SIP beats out PSTN when talking out total volume of calls that can be handled. In particular, unlike traditional PSTN technologies such as ISDN or T1, there are no defined hardware channel limitations on a SIP trunk. SIP requires data capacity rather than physical channels, meaning it can scale to meet your needs as required.

Does a SIP trunk change the customer’s experience?

No! SIP trunk phone systems work very well when they are used with appropriate internet bandwidth and your callers should expect the same great calling experience as traditional PSTN. If you are experiencing lagging or choppy audio, there may be congestion or lowered speeds on the internet service used for your SIP trunk. Your SIP provider can advise you on the expected bandwidth consumption needed to support your call volume.

Good to Know: Glossary of Related Terms

There’s a good deal of technical language that emerges when you dive into the world of SIP trunks. We thought it might be helpful to offer up some textbook definitions of related terms that aren’t quite as familiar as your KPIs. Knowledge is power! (And if nothing else, you can impress your friends or maybe win obscure categories on Jeopardy! with this glossary of telco terms related to SIP trunking.)

A Private Branch Exchange (PBX) is a private telephone system within an enterprise that can handle calls between local, in-system numbers, while also enabling users to call out through external phone lines.

Session Border Controller (SBC) is a network element that compliments SIP and is involved in ensuring the security, connectivity, and service quality of calls.

Interactive Voice Response (IVR) allows callers to interact with computer-based phone systems through voice and by pressing number keys.

An Automatic Call Distributor (ACD) is a technology that distributes calls to agents within an organization, based on customizable parameters such as the caller’s telephone number or their responses to the IVR.

The Public Switched Telephone Network (PSTN) provides infrastructure and services for public telecommunication. The PSTN encompasses the world’s telephone systems, and includes phone lines, fiber optic cables, cellular networks, communications satellites, and undersea telephone cables. By connecting networks to each other and using a single numbering protocol, telephones around the world can call each other through the PSTN.

Voice Over Internet Protocol (VoIP) is a group of technologies that uses internet connections to make and receive voice calls.

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