What is SIP Trunking & How Does it Work With VoIP?

SIP Trunking is an important part of how VoIP phone calls are made. Understanding the tech behind SIP Trunking will help you choose the right phone system for your business.
by
Nicholas Price

Making sense of SIP trunking is helpful if your business is pivoting to VoIP phone communications. Thankfully, it doesn’t take long to understand the basics of this powerful telecom technology and how to set it up for your company, despite the somewhat bewildering name.

Below, we cover how SIP trunking came to be, explain the function of SIP trunks, and how Aircall sets it all up for you in no time

But first things first, let’s decipher the tech jargon. 

So What Exactly is SIP Trunking?

In short, a SIP Trunk is the digital equivalent of a phone line. A SIP Trunk replaces a traditional phone cable enabling you to make and receive voice (or video) calls over the internet.

The name stems from the acronym for Session Initiation Protocol (meaning the computer language that sends and receives calls) combined with the word ‘trunk,’ that techies define as ‘a connection between two points.’

Sometimes, the terms VoIP and SIP Trunking are used interchangeably, which isn’t entirely correct. It’s important to understand that whereas VoIP (Voice over Internet Protocol) is a broad term for any internet-based phone service (yes, that includes SIP trunking), SIP is a specific protocol that implements VoIP by creating start and end points, which determine messages during a call.

Before going into the details about how SIP trunks work, let’s jump back to almost half a century ago, and briefly cover the history of business telephony.

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The SIP Trunking Timeline

To understand SIP technology, it’s a good idea to take a look at the evolution of the systems that led to modern SIP trunking. 

The amusingly named Plain Old Telephone Service (POTS), was how businesses (and your grandma) communicated for over 100 years from 1876 until 1988. POTS is still the most widely used telephone connection you can get through your phone line, via electric signals carried by copper cables from your desk phone to the Public Switched Telephone Network (PSTN). These signals hop from network to network until they reach the right ears. 

When the Integrated Services Digital Network (ISDN) entered the scene in 1988, it was a revolution. All of a sudden, all sorts of signals could be transmitted, rather than just voice. Digital broadcasting of video, HD audio and other forms of data became a reality.

Two interfaces, Basic Rate Interface and Primary Rate Interface (BRI and PRI) run on ISDN. Of these, PRI is used by most large companies as it is able to handle more data. PRI works via the traditional system of electrical pulses transmitted through cables, and a PRI circuit can support 23 simultaneous calls (23 voice channels), which is an important detail that we’ll come back to.

The dawn of internet calling began with VoIP in 1995, and SIP trunking was developed soon after, in the early 2000s. Using a special type of media gateway, carriers could now convert signals from fiber optic internet to copper. That meant that SIP technology could be used to direct internet calls through the conventional PSTN networks without the recipient needing to have a data connection. 

Until a decade ago there were still data rooms in offices dedicated to SIP trunking, with teams of technicians that had to manually set up hardwire systems through SS7 and TDM

Moreover, businesses had to juggle multiple contracts with several providers and voice carriers to provide coverage worldwide. Things are vastly different now, with the option to choose just one virtual provider for all your customers, something that Aircall makes easy. 

SIP Phones and SIP Trunks

Now that we’ve defined SIP trunking, let’s dive a little deeper into the technology. 

As mentioned, a SIP trunk enables the transfer of a call from its origin to its destination, either using a traditional landline (PSTN) or the internet as VoIP. The devices used in the process are simply called SIP phones, classified as either ‘hard phones’ or ‘softphones.’ 

A hard phone looks like a normal deskphone, but it uses an internet connection to place calls rather than the physical PSTN cable grid. A softphone is an installed software or app on an existing device, like a computer or smartphone, that acts as an IP phone. The integrated softphone approach of enabling internet calls via a dedicated app is more versatile and streamlined—that’s why we offer it at Aircall.

SIP Trunks are frequently used in offices to make digital calls via a PBX phone system, but SIP also supports transmission of Unified Communications (UC) like video, SMS, and other streams of data. The fact it works within a UC platform is a major advantage of this technology.

If all this still sounds a little confusing, you can rest easy knowing that Aircall can implement SIP trunking tech for your company effortlessly. VoIP phone providers like Aircall enable businesses with softphones and take care of the tech behind SIP trunking without the need for involvement from your IT department.

Which is Better? SIP trunking vs PRI

There are many advantages to using SIP trunking instead of PRI. In fact, using SIP to place calls can end up being 30-40% cheaper than using PRI phones.

In most business use cases, SIP trunking is highly advisable, especially if your company is expanding or experiences peak calling periods. Should you have doubts about whether SIP trunking is right for your business, Aircall can help clear them up. 

Below we outline the four attributes of SIP trunking that are the technology’s largest strengths compared to PRI:

  1. 1. A defining characteristic of SIP trunking is that the technology allows companies to serve multiple ‘local’ lines without needing a physical presence in that location, as is the case with PRI. This opens up new avenues for localized customer service and remote working. Plus, when you decide to move your office to a new location, your registered phone numbers will be unaffected and no new cables have to be installed, saving your business both time and money.
  2. 2. Another leading advantage of SIP trunking is lower line and DID rentals, as well as lower international and national call charges. It not only saves money for the company; customers can be given the option to dial long distance at a low cost, improving customer service greatly.
  3. 3. As mentioned previously, PRI uses 23 channels per physical line. If you expect more than 23 concurrent calls, you’ll need to install more lines. This can be inefficient if you need, for example, 25 channels but have to pay for two lines (46 channels, of which most go unused). As you can imagine, this solution does not easily scale with your business. With SIP trunking, you have complete control over the number of phone lines you require, only paying for what you need. If you want to add or remove channels this is a very quick process (Aircall makes adding new phone numbers easy within the app). Gone are the days of waiting for a technician to arrive and tinker with cables.
  4. 4. SIP trunking streamlines business operations. It eliminates VoIP gateways, uses modern IP PBX, offers better quality and can integrate seamlessly with CRM platforms. Virtual SIP trunking provides a user-friendly admin portal making management easier—functions that PRI can’t handle.

If your company doesn’t want to completely forego PRI for any reason, a hybrid solution does exist where PRI is used in conjunction with SIP trunking. However, the majority of organizations today have already adopted or plan to switch to SIP trunking for the cost savings and business agility it affords.

Choosing a SIP Trunking Provider

If you’re convinced SIP trunking is the way to go, you’re probably asking yourself “How do I choose a SIP trunking provider?

Aircall makes it simple. We use a behind the scenes BYOC (Bring Your Own Carrier) model to work together with the best providers to ensure quality coverage across the world. This way, your IT team won’t need to juggle multiple contracts, since it’s all handled by Aircall.

Aircall is completely cloud based, making desk phones (SIP ‘hard’ phones) obsolete. We use softphone WebRTC technology instead, which allows callers to access voice transmission through web browsers, web apps, and mobile apps (such as iPhone and Android devices). In short; Aircall connects to the cloud, the cloud connects to a user’s device (computer or smartphone), and the app facilitates SIP trunking through the best suited provider.

How to Make the Switch to SIP Trunking

SIP trunking technology is a major boost to business telecom operations. If the actual process still seems a bit cloudy, don’t worry! Get in touch and we’ll handle the rest.

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