SIP trunks: What every business needs to know

man using an office phone
(Image credit: Shutterstock)

SIP trunking is a type of protocol used for VoIP, although the relationship between the two has often been confused. In this article, we’ll take a close look at what exactly SIP is, what SIP trunks are, and what the related benefits and limitations for businesses might be.

As we’ll see, SIP trunking provides a robust, scalable, and secure protocol for voice and other communications, which is one of the reasons why many of the best VoIP providers choose to use this protocol.

What is SIP?

SIP stands for session initiation protocol and is a popular method for sending and receiving communications data over the internet, including voice and video calls. It’s used to establish, maintain, and terminate calls in VoIP applications. For example, services like Skype and Zoom that enable you to call another user or even a regular phone line over the internet use SIP to make this possible.

A common question among businesses looking for the best way to unify communications and reduce costs relates to the differences between SIP and VoIP, but this is actually a faulty comparison. SIP and VoIP are not two distinct protocols to be compared. Rather, SIP is just one of several protocols that VoIP providers use to manage calls.

A “trunk” is a technical term in telephony that refers to a collection of phone lines, although the definition has now grown to include virtual connections like those used by SIP. 

SIP trunks replace the copper wire that physically connects machines on a traditional public switched telephone network (PSTN), instead sending information digitally over the Internet. In other words, SIP trunks provide the means for companies to send voice and other communications information over the Internet, both internally and externally.

SIP trunks can also carry text messages, emails, and even internet access, and for this reason are often employed in unified communications (UC).

Centralized vs. decentralized SIP trunks

There are two basic ways to deploy SIP trunks: centralized and decentralized. In a centralized approach, all calls are directed to a single hub (a company’s headquarters, for example), and then routed internally to different locations as required. The advantage is having a single, unified system between multiple locations. 

In decentralized SIP trunking, each location or department has its own SIP trunk and manages calls independently. While this is costlier, it is also more robust, as loss of service at a single SIP trunk won’t bring down the whole network.

How can your business benefit from SIP trunks?

A business can benefit from SIP in a number of ways.

SIP, and VoIP in general, is typically much cheaper than traditional telephony. Because the data is travelling over the internet, you no longer have to pay per call (much like how you don’t pay per email or instant message), and long-distance calls are much cheaper as they’re essentially treated like local calls. In addition, you only need to pay for an internet connection rather than internet and (physical) phone lines. 

SIP trunks are also made available in smaller denominations than traditional phone lines, whose trunks are typically offered in groups of 12. In the past, if you needed 14 trunks, you’d have to pay for 24. SIP trunks are often sold in smaller packs, or even individually.

SIP providers enable businesses to unify many of their communications: email, text messaging, voice and video chat, voicemail, digital faxing, and more, all run through a single system and off a single platform or software suite. In general, SIP and other UC platforms offer a simpler, cheaper solution for internal and external communications, with a quick ROI.

man in office on a phone call

SIP can help you unify office communications. (Image credit: Shutterstock)

SIP trunks can be built with the same encryptions as used for other internet protocols, like TLS, which greatly improves the security of voice and other communications. This kind of encryption isn’t possible over traditional phone lines.

SIP and VoIP are also more resistant to disasters, both natural and technical. Redundancy, back-up and fall-back systems, rerouting calls, and taking advantage of decentralized SIP trunking all make it less likely your business will have to face significant downtime.

An unlimited number of channels (incoming or outgoing calls) can pass through each SIP trunk; the only limitation is bandwidth. As a result, it’s much easier (and cheaper) to scale communications when using SIP or other VoIP solutions than with traditional phone lines.

This also results in greater agility, as a business can scale up during communications spikes, and scale back down afterwards. 

Because audio, video, and other information is passed over the internet, SIP trunking provides a more flexible means of communication for employees who choose to use their own devices (BYOD) or work from home. Part- and full-time remote workers can be managed much more easily than with traditional telephone lines. 

SIP trunking makes it easy to provide local numbers around the world, even if you have a single location or PBX, providing a more comfortable experience for your customers than 800 numbers. It’s also a much cheaper solution.

SIP trunking also makes it easier and faster to redirect calls, a process which can be automated for an even smoother customer experience.

What limitations are there with SIP trunks

screenshot of google dinosaur game

One downside to SIP is if your internet goes down, so do your phones (Image credit: Google)

There are some limitations associated with SIP trunks compared to other protocols and traditional telephony.

First, because all of your communications are now internet-based, if there’s a problem with your internet connection, your phone lines will be out too. The same goes for general power outages. That being said, VoIP and UC providers offer many solutions, like rerouting calls to mobile devices or other offices, something that’s more difficult with traditional lines. 

Businesses should also be aware of certain security concerns. While, as mentioned previously, SIP trunks can be encrypted in a way traditional telephone lines can’t be, if a hacker gains access to your servers, they will also have access to your communications.

VoIP and UC as a Service (UCaaS) providers often employ robust cybersecurity solutions, however, and it’s incorrect to say that SIP trunks are more vulnerable than other solutions. In general, they are a safer solution—just know that vulnerabilities do exist and have to be taken into consideration.

Finally, as your messages and calls now go through the internet, you’ll need to make sure you have sufficient bandwidth, and you may need to opt for a more expensive plan (although, in the end, it will still be cheaper than traditional telephony).


SIP trunks provide a flexible, cheap, and reliable alternative to traditional phone lines, and can be used as part of a robust VoIP or UC approach to internal and external communications. SIP trunking reduces cost per call, especially that of long distance calls, and provides a single platform for audio, video, and other communications. 

Despite a few limitations, SIP trunking is a method employed by many of the best VoIP providers, and offers many benefits to businesses that choose to use this protocol.

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Christian Rigg

Christian is a freelance writer and content project manager with 6+ years' experience writing and leading teams in finance and technology for some of the world's largest online publishers, including TechRadar and Tom's Guide.