SIP • Last Updated 9/29/2023

The Real Difference When it Comes to SIP Trunk vs PRI

The right phone system can make or break how you do business. When it comes to SIP Trunk vs PRI, there's a clear cut winner.

By Brian Segal

PRI phone wires vs SIP trunking

Choosing a new phone system may seem fraught with acronyms and unfamiliar technology. However, the array of acronyms and tech comes down to comparing two key systems: PRI vs SIP trunk.

This comparison can be summed up as classic versus modern. Here’s how to determine which is best for your business.

PRI vs SIP: What’s the Difference?

PRI (Primary Rate Interface) trunking and SIP (Session Initiation Protocol) trunking are both systems for connecting calls to your regional telephone network. But while PRI is based on traditional telephone networking technology, SIP trunking is built on internet connections.

This means that PRI relies almost entirely on physical wire connections. Of course, a SIP trunk requires some physical connection points but much of the data transmission in SIP trunking goes through virtual connections. As you’ll see later on, this difference has big impacts on the flexibility and scalability of each system.

Now, both PRI and SIP use a PBX (Private Branch Exchange) system to connect the lines in your building to the outside public phone lines. And, with the right adapters and hardware, you can use the same phones for PRI and SIP trunking connections.

So, in the PRI vs SIP trunk comparison, the major differentiator is how call data is moved from one end of the call to the other. This difference presents certain advantages and disadvantages for each system.

What is PRI?

A PRI (Primary Rate Interface) trunk is a packaged bundle of wires that carries up to 23 voice channels. One PRI trunk can have up to 100 phone numbers, but can only handle 23 simultaneous calls. A PRI trunk is dedicated to phone calls, and can only transmit voice signals.

PRI is the traditional connection method associated with the PSTN (Public Switched Telephone Network). It’s a very straightforward connection method that essentially creates a copper wire connection from one end of a phone call to the other, with the PSTN in the middle.

The 23 voice channels in a PRI trunk can be reconfigured and assigned to new phone numbers relatively easily. So PRI offers some flexibility for businesses that operate in large office buildings or need to establish connections for special needs like video conferences.

Benefits of PRI

The benefits of PRI are dervived from the infrastructure of a physical, copper wire connection. Let's dig into the key benefits:

  • Good call quality and stability: Since the connection is established on a physical wire, the call quality rarely changes. Unless something disturbs the physical networks that connect the call (which is unlikely), you get a consistent high quality audio signal and a dependable connection.

  • Traceable calls: Calls on a PRI trunk can be traced pretty easily. This is useful for 911 calls, since it’s easy to route the call to the nearest dispatch center and establish a location to send emergency responders.

  • Secure calling: It’s not easy to intercept the signal on a PRI trunk. Unless the call is transferred to an internet connection at some point, a person would need to tap into the copper wire to listen in on a call on a PRI trunk. In short, the potential for intercepted calls on a PRI trunk is very minimal.

Obviously, PRI trunking works. It’s not a bad option. But it’s also not perfect.

Drawbacks of PRI

Although copper wires have their benefits, the main limitations of PRI are also related to its dependence on physical connections. Watch out for these limitations, if you’re running calls on a PRI trunk:

  • Difficult to scale: If you want to handle more than 23 simultaneous calls or 100 phone numbers, you’ll have to add another PRI trunk, which means running another pack of cables. When you expand your PRI phone system, you’ll have to add 23 connections and 100 phone numbers at a time and it requires a good bit of physical labor to get each new cable pack installed.

  • Not super flexible: Yes, phone numbers on a PRI trunk can be reassigned. However, it’s a more involved process, since it requires managing physical connections. You’ll need patience if you often need to reassign phone numbers.

  • Limited voice channels: As we mentioned earlier, a single PRI trunk is only capable of handling 23 simultaneous voice calls. That means that you may have to add additional PRI trunks to meet your simultaneous call volume, even if you have less than 100 phone numbers. This can be especially limiting for call centers and other businesses that need to make a lot of concurrent calls.

  • Can be expensive: Unfortunately, it can be costly to run new cables. So it can be expensive to expand a phone system built on a PRI trunk. On the bright side, most of the costs are one-time expenses associated with building the physical infrastructure for a PRI trunk.

Ultimately, PRI offers good call quality and security. But it’s best for businesses that can work with a relatively fixed phone infrastructure and limited simultaneous calls, as flexibility and scalability aren't exactly the strong points of PRI trunking.

If that doesn’t sound like exactly what you need, SIP trunking may be a better solution - after all, it’s much easier to reconfigure virtual connections.

What is SIP Trunking?

SIP (Session Initiation Protocol) trunking is an alternative to PRI that’s designed to offer better scalability and flexibility, with minimal (or no) tradeoffs in call quality and security. SIP trunking replaces the physical bundle of wires in a PRI trunk with internet connections.

When a call is connected through a SIP trunk, the phone systems send signals to each other that indicate when to start and end the call, play-back ringing sounds, and stream the audio data between the two phones on the call.

Although the SIP trunk initiates and terminates calls, the audio signals are transmitted using internet protocols—either UDP (User Datagram Protocol) or TCP (Transmission Control Protocol). Most of the time, your calls will connect through your internet service provider’s IP network.

Benefits of SIP Trunk

Relying on virtual connections rather than physical connections adds a ton of flexibility and scalability. But what exactly does this mean? Here’s how connecting calls through SIP trunking can offer a better experience:

  • More simultaneous connections: With a PRI trunk you must add another cable bundle if you need more than 23 simultaneous voice calls. A SIP trunk doesn’t have this problem. You can make as many simultaneous calls as you need, so long as your internet connection has enough bandwidth to carry all the voice data. This makes SIP trunking ideal for any business that needs to make a lot of calls at once, like telemarketing firms and customer service call centers.

  • Easy to scale: Since virtual connections are easier to establish, it’s much easier to scale your SIP trunking telephone infrastructure up and down. Need more phone numbers? You can provision them in your web browser. On the other hand, if you need to get rid of phone numbers you can release them just as easily, and there’s no phone cable left behind.

  • Excellent call quality and reliability: Call clarity and reliability is one of the main concerns with using the internet to make phone calls. However, you can get outstanding call quality with a broadband connection and the right telecom carrier. In most cases, you won’t be able to tell the difference between a call on a PRI trunk and a call on a SIP trunk. So you get a more scalable and flexible phone system, with almost no noticeable drop in call quality and reliability.

  • Cost efficient: Since there’s no cable to run for adding new SIP trunking connections, expanding your phone infrastructure is very cost efficient, with relatively little upfront cost. Additionally, new phone numbers are affordable and calling costs are low because the calls connect through the internet service you’re likely already paying for.

Overall, SIP trunking makes it easy and affordable to build your telephony infrastructure and supply all the phone numbers and connections you need.

Drawbacks of SIP Trunk

Even though flexibility and affordability are great, SIP trunking isn’t quite perfect. Here are some limitations to be aware of.

  • Limited call location services: Although SIP trunk connections can be configured for call tracing, virtual connections make it more difficult to pinpoint exactly where a call is coming from. In some use cases, this isn’t an issue. But, if you need to trace a call — to get a faster response to a 911 call, for instance — it’s not as easy to do with a SIP connection. There have, however, been developments addressing this issue.

  • Potential call quality issues: Call quality on a PRI trunk is very stable, since there’s very little variance in the physical connection. There are more variables involved with sending a digital audio signal over an internet connection. So it’s possible that you may experience degraded audio quality if there are issues with your internet connection or your telecom carrier’s network. However, if you work with a tier 1 carrier like Telnyx, these call quality issues should be minimal or even nonexistent.

  • More security considerations: One of the biggest challenges with SIP trunking is that transmitting calls over the internet opens calls to potential cyberattacks. Now, there are ways to secure calls, and a quality telecom carrier will use end-to-end encryption and a private network to protect call data and minimize the potential for cyberattacks. But it’s important to take the security considerations of the internet into account when you use SIP trunking.

Fortunately, there are several SIP trunking technologies — fixed VoIP numbers, private carrier networks, and end-to-end encryption — that enable you to use SIP trunking without really feeling the effects of these potential drawbacks.

Which is Best for Your Business?

When you add everything up, the end user usually won’t be able to tell the difference between calls made on a PRI trunk or those connected with SIP trunking. The differences between the two connection methods are most apparent on the backend, for the individual or business that purchased the connection.

There are two primary factors that will determine if you should use PRI or SIP trunking:

How many simultaneous calls you need to make.

If you need to make more than 23 simultaneous calls, consider SIP trunking. Using a PRI trunk can be expensive and slow to implement, since you’ll need to install several PRI trunk cable bundles to get the voice channels you need. Also, if you need to expand your simultaneous call capabilities, it will require another expensive and time consuming installation. Contrarily, the only thing that limits the number of simultaneous calls you can make with SIP trunking is your internet bandwidth. If you have enough bandwidth, you can keep adding calls to the connection without any call quality issues.

How often you need to get new phone numbers or reassign phone numbers to new devices.

You get 100 potential phone numbers with a single PRI trunk and if you need more then you’ll have to go through the process of scaling up your PRI infrastructure, which (again) can be time consuming and cost prohibitive. So if you need more than 100 numbers, SIP trunking will likely be a better option as it’s much easier to add new phone numbers and reassign phone numbers to new devices with SIP trunking. A good SIP trunking provider will also have a simple, unified interface for provisioning and setting up phone numbers.

The bottom line is this: if your use case requires making many simultaneous calls, supporting a large inventory of phone numbers, or frequently provisioning or reassigning new phone numbers, you should look into SIP Trunking. It’s much easier to establish the infrastructure and spin up the connections for high call volumes and many phone numbers.

Conversely, PRI will work well enough for businesses that can operate using a fairly static telephone infrastructure, with modest simultaneous call volume and required phone numbers.

Talk to a Telnyx Expert

Does SIP trunking sound like a good fit for your business? Telnyx delivers SIP trunking on a private IP network that gives you all the flexibility, scalability, and affordability of SIP connections, with the same (or better) call quality and reliability you’d get from a PRI connection. Set up SIP Trunking with Telnyx to make and receive secure, highly configurable calls with carrier grade quality and global reach.

Get in touch with a Telnyx expert to get answers to all your SIP trunking questions.

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