What is SIP Trunking: Your Questions Answered

Call Center | 5 minute read

Contact centers’ conversion to SIP trunking technology has been an ongoing industry success story. Why? For starters, using SIP technology to enable voice and video calls, messaging and data transmission saves money: up to 50% according to Gartner analysts. And SIP trunking is super-flexible; it makes it easy for contact centers to add advanced features like speech analytics, 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 break it down, it’s helpful to think of a SIP trunk as the virtual version of an analog phone line. SIP stands for session internet protocol, and technically speaking, it’s a signaling protocol that initiates interactive user sessions over data connections. A trunk is a circuit between two switching systems that carries a certain number of calls at a time.  Every call center has a trunk to carry its phone calls: a SIP-enabled trunk lets you use an internet connection instead of a traditional phone line to carry voice and video calls and other data transmissions..  


The “trunk” in SIP trunk refers to a virtual connection and is not a physical line. The word 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 the business from the carrier. Because a SIP trunk delivers the same functionality, the name stuck.

What are the benefits of SIP trunking?

SIP trunking is popular for a reason. There are many benefits of using the technology including the fact that it allows you to use the same IP-based network for a variety of data transmission, including voice and video calls. SIP trunking eliminates the need for multiple networks and also frees you from having different subscription services for voice and data. It lowers your hardware costs, too, 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. As we mentioned earlier, it’s up to 50% cheaper than traditional telco setups and easily facilitates advanced features like Visual IVR and speech analytics.

In fact, this inherent flexibility is one of the biggest advantages of SIP. Because SIP enables voice and data through standard protocols, external vendors can interact with your communications flow without having to create proprietary “adaptors” for each call center platform. 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 make a great team, but they are not the same thing.  VoIP enables voice calls over the internet.  SIP is a signaling protocol that initiates, changes, or cancels multimedia communications – including VoIP. SIP enhances VoIP because it allows you to transmit video, files, and messages, as well as voice.

How many channels are in a SIP trunk?

There is not a set number of channels in a SIP trunk because the number of channels grows and shrinks when your agents connect and disconnect calls. When you have 20 calls in progress, your SIP trunk provides 20 channels to accommodate these calls.

A better question is: How much capacity is in your SIP trunk?  You see, when it comes to SIP trunking, we’re really talking about data capacity rather than physical channels. Typically, the data capacity is sold as “channels” to make it easier to package and sell.  A SIP trunk can have as many channels as you need, but you’ll need to discuss the capacity your contact center requires with your SIP provider.

So, how many SIP trunks do I need?

Again, what we’re really talking about is capacity. A SIP trunk can accommodate as many channels as you need if you provide enough bandwidth. So really, you only need one.

Does a SIP trunk change the customer’s experience?

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 they have with other telephony systems. If you are experiencing lagging calls or choppy audio, that likely means you lack the bandwidth to support your SIP trunking set-up, or you have a bad internet connection. Your SIP provider can let you know the exact bandwidth needed to support your system and usually, upgrading is easy and not cost-prohibitive.

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 contact center 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.)

Multiprotocol Label Switching (MPLS) is a networking technology that routes traffic from one node to another based on labels rather than network addresses.

Private Branch Exchange (PBX) is a telephone system within an enterprise that switches calls between users on local lines while enabling all users to share a certain number of external phone lines.

Public Switched Telephone Network (PSTN) provides infrastructure and services for public telecommunication. Originally a network of fixed-line analog telephone systems, the PSTN is a network of the world’s circuit-switched telephone systems. These systems include phone lines, fiber optic cables, cellular networks, communications satellites, and undersea telephone cables. By interconnecting their networks and adopting a single numbering protocol, telephones around the world can phone each other.

Time Division Multiplexing (TDM) is an older technology that uses physical switches to route telephone calls over copper wires.

Voice Over Internet Protocol (VoIP) is a phone system that uses an internet connection to make and receive voice calls.

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