What Is PRI?

August 31, 2018 | 13 min read

what is PRI (primary rate interface)

What is PRI? A PRI – or Primary Rate Interface – is an end-to-end, digital telecommunications connection that allows for 23 concurrent transmissions of voice, data, or video traffic between the network and the user. The PRI line, or circuit, is a physical piece of equipment.
PRI technology has been around since the 1980s. It was one of the most widely used technologies for business communications, though use of this type of system has declined in recent years due to the growing popularity of VoIP.

VoIP market growth projections
We’ll cover VoIP and other alternatives later in the article. To skip to that or any other section in the article simply click on the table of contents below. Otherwise, continue reading to learn exactly what PRI is.

Table of Contents

Features of a PRI line

To get a better understanding of what a PRI does and how it actually works, let’s start by taking a look at some of its key features:

  • A PRI line is made up of two pairs of copper wires connecting the service provider (network) to the customer (business end-user).
  • A PRI is technically one line, but it can simultaneously transmit 23 separate communications (voice, data, or video.) This means up to 23 people in your company can be on the phone (or sending an IM or having a video chat) at the same time.
  • Each of a PRI’s channels has a 64 kbps capacity for data transmissions.
  • PRI lines can work with an analog or mixed EPABX setup as well as an IP PBX system.
  • A PRI can join two PBX systems together, increasing the number of available communication channels.

PRI T1 vs E1

The PRI circuit – the actual cable that physically connects your telecom system – is available in two flavors: T1 and E1. The main difference between the two PRI circuits has a lot to do with geographical location, though there are some technological differences to be aware of:



Used in Europe and AustraliaUsed in North America and Asia
Provides 64 Kpbs for data transmissionProvides 64 Kpbs for data transmission
Transmit and receive rate of 2.048 MbpsTransmit and receive rate of 1.544 Mbps
Physically contains two pairs of copper wiresPhysically contains two pairs of copper wires
32 total channels24 total channels
30 channels used for voice, data, video23 channels used for voice, data, video
2 channels used for signaling1 channel used for signaling

Functionality of PRI

If you’ve called a business anytime in the last decade, there’s a good chance you had some interaction with a primary rate interface (PRI). Let’s take a step back and get a view of how the PRI works in a business setting.

Direct inward dialing (DID)

how DID works with either PRI or SIP
Generally, most people within a company have their own direct number, meaning someone from the outside can dial the number and reach them directly. (Note that there are exceptions to this, of course, like in a call center setting)
But each of these individual phone numbers does not mean there is a literal separate phone line connected to each person. Not at all, in fact. And this is where the primary rate interface comes in.
You’ll remember we mentioned a PRI can handle up to 23 conversations (IM’s, videos, etc.) simultaneously. So, the idea is one PRI can be set up for 23 users in a company. However, small to medium-sized businesses bank on the fact that it’s unlikely those 23 people will actually be on a call (or chat, or whatever) all at the same time.
With this in mind, it’s common to see up to 50 users assigned to one PRI. That means 50 different phone numbers, all running through the same PRI circuit, with the capability of 23 communications happening at the same time.


Now as a business grows and more users are added, it’s a lot more likely to have 23 or more communications happening simultaneously.
In this case, an additional PRI circuit can be purchased and added to the existing system, opening up the ability for another 23 channels for voice, data, or video transfers to happen.

Alternatives to PRI

There’s no denying primary rate interface technology changed the way businesses could communicate with customers… 30+ years ago. But since that time, advances in telecommunications technology has shined a pretty bright light on some of PRI’s shortcomings.


In the US, a primary rate interface gives you a chunk of 23 channels to work with. There’s no option to add one or two channels as a business slowly grows. Likewise, if a business rapidly increases in size or takes on a large contract, adding more channels still means doing it in blocks of 23.
Consider a small business. A five or six-person company just doesn’t need 23 communication channels. In this case, the company is paying for a lot more than they actually need. Think of it like buying a 15-passenger van for a family of four.
On the flipside, a medium-sized business or large corporation faces different challenges with PRI. Adding a large number of users requires purchases multiple PRIs (again, in chunks of 23 channels). Setting up multiple locations can be a headache. And remote workers? Forget about it.

Other options

PRI alternatives
More and more, businesses are looking for alternatives to PRI. There are several options out there, but not every option is a good fit for every business. Here’s a look at some other solutions.

Analog POTS lines

POTS = Plain old telephone service. We’re talking about the old school phone line that used to connect every single house and business. Your grandparents had one. Your parents had one,. You might even have POTS at your house. In the residential space, POTS is often referred to as a landline or a PSTN.
POTS provides service through a single analog circuit connecting service provider and the user. A POTS supports once communication at a time. This means a single phone call renders the line unavailable for anything else, like a fax or modem internet access.
POTS diagram
At one point in history, analog POTS lines were the way people communicated by phone. And it could still work as a solution for a small business today, if phone traffic is minimal.
POTS is generally very reliable, but the technology has reached its apex. As other communications systems develop, we’ll begin to see POTS disappear from the scene.

SIP Trunking

Session Initiation Protocol – or SIP – is the method of transmitting a voice communication over a data network. A voice over IP (VoIP) call is an example of a SIP session.
SIP trunking is sort of like a POTS (remember – plain old telephone service). But, with SIP trunking there are no phone lines running all over the place. Instead, the ‘lines’ are virtual because the phone system is connected to the service provider through your network (your internet connection).
sip trunking diagram
In the past, SIP trunking has been frequently used as a replacement for POTS or PRI systems. Depending on business size, it may still be a viable option.

Hosted VoIP

Today, many businesses of all sizes are moving to a hosted VoIP solution. A hosted VoIP – also known as a hosted PBX – simply means everything is hosted off-site. The provider hosts the service, deals with the servers and software, and handles maintenance, updates, and troubleshooting.
The service provider manages all of the backend work (whether it be calls, chats, video conferences, etc.) and routes them to the business’s existing phone system.
how VoIP works diagram
The big advantage of a host VoIP system is apparent. The business itself doesn’t have to worry about expensive equipment, installing and maintaining a big server, or upgrading to the latest software. These things are taken care of by the service provider, saving the business a lot of money, manpower, and headache.

Comparison: Hosted VoIP vs PRI vs SIP trunking

Let’s take a closer look at the most popular telecommunications solutions on the market today: VoIP, PRI, and SIP trunking. How do the three stack up? We’ll compare each using this criterion.

  • Startup costs
  • Ongoing and maintenance costs
  • Connectivity
  • Backup and redundancy
  • Business continuity
  • Flexibility and upscaling
  • Quality of service

Startup costs

It’s doubtful you’ll find any phone system that doesn’t require some sort of upfront investment. The amount of this investment varies widely depending on the system you choose.

Hosted VoIP

You’ll need a few things to get a hosted VoIP system up and running – VoIP phones, routers, and high speed (business grade) broadband internet service. Everything else, like servers and software, is handled by the service provider.
Most organizations already have business grade broadband service, so they’re left with purchasing IP phones and routers. This equipment is relatively affordable and, in some cases, can even be rented from the provider. This option saves you even more upfront.
Hosted VoIP upfront costs: Low
SIP Trunking
SIP trunking requires an on-site SIP server. Naturally, the server will require installation and maintenance. Additionally, many businesses opt to utilize a separate internet connection for their SIP trunking, adding another monthly broadband bill.
The cost of the server itself, along with its ongoing costs, can be significant. To better understand just how much of a difference this can be compared to hosted VoIP, we created a pricing chart below.
on-premise PBX initial costs are much higher than hosted VoIP or PRI
SIP trunk upfront costs: High


PRI circuits are physical connections, meaning there’s onsite installation required. Each group of 23 channels required will lead to another PRI circuit installation. If your business requires many communication channels, this could lead to high installation costs.
If the PRI phone system will be used with an IP PBX, a primary rate interface card will be required. This can increase upfront costs significantly.
PRI circuit upfront costs: Medium to high

Ongoing costs

Upfront investment isn’t the only cost associated with a phone system. Ongoing costs, whether monthly service fees or maintenance, can quickly impact your ROI.

Hosted VoIP

Hosted VoIP systems generally work on a monthly subscription fee model. This could be a per user/per month fee or a block of users/per month (eg. 0-10 users/$xx per month.)
Pricing can also depend on the feature packages, offering more communication features for a higher monthly fee.
In some cases, hosted VoIP service may also be charged at a straight per minute rate.
For the best rates combined with the best solution, check out Nextiva’s three commercial phone service packages.
Hosted VoIP ongoing costs: Varied; Low – medium

SIP Trunking

Pricing for SIP trunking is often handled per channel, where one channel = one call. This means businesses must examine how many simultaneous channels they’ll need at a given time. If 25 people could potentially be on the phone at the same time, they’ll need at least 25 channels.
The charge per channel is usually a flat per month rate. For up-to-date pricing click here.
Let’s think back to the previous pricing chart and see how much of a difference this type of on-premise system costs a company over the course of a couple years.

on-site PBX can cost more than hosted VoIP or PRI over a 2 year period

SIP trunk ongoing costs: Varied; Medium – high


PRI circuits come along with a monthly fee. This is in addition to any onsite maintenance or troubleshooting, the cost of which will fall to the business.
Long distance and internationally calling is also relatively expensive with a PRI system. Depending on the business, this could be a significant expense.
PRI ongoing costs: Medium – high


As we’ve demonstrated, different telecommunications solutions require different kinds of hardware and software. Thus, each system’s connection differs from the others.

Hosted VoIP

A hosted VoIP’s connection is completely virtual. There are no phone cables connecting the business to the service provider or connecting the business to an outside phone line. The name itself explains it all – voice over IP (internet).
With hosted VoIP, all the wires, cables, and cords are handled by the service provider as the host.
Hosted VoIP connectivity: Virtual

SIP Trunking

Likewise, SIP trunking also works as a virtual connection over a business’s internet connection. The SIP connection eliminates the need for analog phone lines.
SIP trunk connectivity: Virtual


PRI circuits are physical pieces of hardware. These require installation on site, and, as you’ll remember, each circuit provides 23 channels. If a business needs more than 23 channels then they’ll need to install multiple circuits.
PRI connectivity: Physical

Backup and redundancy

Think of backup and redundancy as ‘what happens in a disaster’. Whether it’s something simple like a power outage or something catastrophic like a tornado, businesses require a system that can support their needs during a disaster.

Hosted VoIP

The fully virtual nature of a hosted VoIP system is well suited for any sort of disaster plan. Users can essentially log in from anywhere if the physical office isn’t suitable. If this isn’t an option, calls can easily be forwarded to a third-party, either by the host or by the business.
VoIP service providers also plan for disasters or downtime by setting up multiple host sites. If one site is down, service and hosting can easily be rolled to another site.
Let’s take a look at Nextiva’s connection history, for example. Below is a log of Nextiva’s uptime over the past 90 days. To see this data in real time simply go to status.nextiva.com.

VoIP is more reliable than PRI

Hosted VoIP backup capability: Strong

SIP Trunking

Because SIP service is virtual, it too can be handled efficiently in a disaster situation. Communications can be pointed to another geographic location if the business has lost connectivity.
SIP trunk backup capability: Strong


PRI circuits are physical, so a disaster on site can cause substantial issues. Businesses can build in extra PRI circuits for redundancy sake, but this can carry a hefty price tag.
PRI backup capability: Weak

Business continuity

Business doesn’t stop just because the power goes out or you’re in the middle of a snow storm. Let’s examine how (or if) business can carry on even when disaster strikes.

Hosted VoIP

As a 100% cloud-based system, communications can be transferred anywhere at any time.
Hosted VoIP continuity level: Very high

SIP Trunking

In the event of an emergency, SIP trunking supports the transfer of calls, but only to predefined locations.
SIP trunk continuity level: Medium


The physical nature of PRI circuits does not allow for call rerouting in the event of an emergency.
PRI continuity level: Low/none

Flexibility and upscaling

Planning for the future growth of a company is imperative when selecting a phone system. The solution should not only fit the business’s current needs, but also it’s forecasted goals and future needs. The ability to scale a phone system quickly and easily is important.

Hosted VoIP

When it comes to flexibility and quick scaling, the best system available is a hosted VoIP. Deployment of a hosted VoIP can happen in a few short days.
Adding additional users, whether it’s a few or a few hundred, is as simple as entering users in the system and contacting the host to scale up the monthly plan.
Hosted VoIP flexibility: Very high

SIP Trunking

Growing your SIP-enabled system will require your service provider adding more SIP trunks. While the process is relatively easy, speed will depend on your service provider’s responsiveness. SIP systems can be scaled by a few or by many users since each trunk is an individual communications channel.
SIP trunk flexibility: High


Scaling a PRI system requires physical installation of additional PRI circuits. This can be costly, depending on the scope of work. Scaling is limited to groups of 23 channels, so if you need only two or three more users, you’ll still be required to have an entirely new/additional PRI circuit added.
PRI flexibility: Low

Quality of service

The cheapest, most flexible phone system doesn’t matter if the quality of phone calls is terrible. Because of the differences in technology, call quality and service can vary a great deal among the various phone solutions.

Hosted VoIP

VoIP call quality depends largely on the condition of the internet service it’s using. Low bandwidth or poor connection can result in VoIP jitter, packet loss, or delays, which negatively affect the quality of the call itself.
A strong internet connection, however typically results in excellent, HD quality of service for VoIP calls.

voip jitter buffer bell curve

Hosted VoIP quality: High

SIP Trunking

SIP systems can suffer from the same issues as a hosted VoIP, such as jitter or delays. Likewise, a strong internet connection can alleviate these problems, providing excellent quality.
SIP trunk quality: High


Calls transmitted through a PRI circuit will sound similar to calls made on a POTS line. The frequency range carried by PRI circuits is extremely limited compared to it’s VoIP and SIP counterparts, which transmit calls in HD.
Calls made on a PRI may sound somewhat muffled or distant compared to the clear, crisp sound heard through hosted VoIP or SIP trunks.
PRI quality: Low

Final thoughts: The best option

We’ve answered the question, “What is PRI?” And we know a little about how it works, its benefits, and drawbacks. Can we confidently decide PRI is the best phone solution?
Probably not.

SIP Trunking

A SIP trunk certainly has advantages over PRI, if only for the ease of scalability and call quality. Installing a SIP trunk is pretty easy (depending on you current PBX), and your business won’t be burdened with a ton of phone wires.
Though there’s a hefty upfront cost, SIP trunking might be a good option for some businesses.

Hosted VoIP

Better still is the hosted VoIP system. Cost, ease of deployment, scalability, and disaster recovery are all spot on with this solution.
What else makes hosted VoIP so great? A few things, actually.

  • It’s simple to manage. Getting started is simple, and ongoing management is a breeze. You don’t need to be an IT professional to set up new users. Everything is usually done in a simple dashboard.
  • You can add or remove features. Hosted VoIP systems make it easy to tack on options, like chat or video conferencing.
  • It’s scalable. A hosted VoIP system can easily be scaled from one user to 500+. It’s truly as easy as adding the users and upgrading your monthly plan. (Excellent for call centers!)
  • It’s mobile. Hosted VoIP systems support all manner of mobile communications. This cuts the tie between your employee and his desk.

As a frontrunner in the hosted VoIP space, Nextiva offers one of the world’s best business communication systems.  Here are a couple of the products that can take your business to the next level.

  • Nextiva OfficeConnect your employees and your customers – anywhere. With the country’s most reliable, enterprise-grade business phone service you’ll never miss a call, IM, or video conference.

  • Nextiva Contact CenterLooking to take your call center to the next level? Give your agents the best chance to help your customers by setting up Nextiva’s call center solution. As an omni-channel communications platform, everything your agents needs is right at their fingertips. No more scrolling through pages of information or toggling back and forth between a bunch of windows. Your agents can focus on what’s important – Your customer.
  • Nextiva SIP Trunking – If you’d rather go the SIP trunking route – Nextiva can help with that as well. Nextiva SIP Trunking offers all-inclusive pricing so you don’t have to worry about per minute charges. You know the costs upfront.

Try Nextiva

Give us a chance to show you what amazing service really means. Reach out to us to ask about any of our products. If you’re not sure what you need, reach out to us anyway! We’d love to learn about your business and how we can help.

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Cameron Johnson


Cameron Johnson

Cameron Johnson is a market segment leader at Nextiva. Along with his well-researched contributions to the Nextiva Blog, Cameron has written for a variety of publications including Inc. and Business.com. Cameron was recently recognized as Utah's Marketer of the Year.

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