ISDN PRI vs SIP Trunking: What’s the Difference?

Posted on: 2019-02-28 | Categories: SIP Trunking VoIP Technology

Are you looking to purchase a new phone system for your business? You’re probably drowning in a sea of jargon then. Some people will tell you PRI is better while other vehemently support SIP. Reality is that both have their pros and cons. What’s best for your business comes down to what you need and can afford.

What is ISDN PRI?

PRI or Primary Rate Interface is a telephone technology that delivers 23 voice channels and one data channel to companies over an ISDN (Integrated Services Digital Network). PRI uses T1 lines to deliver multiple channels on a single circuit. The technology has been around for more than 40 years. It is a physical line connecting a business to the phone service provider.

What is SIP Trunking?

SIP is a network technology, it’s not telephony based like the PRI. A SIP trunk is a virtual line, you don’t need any hardware. There’s no installation or maintenance needed. SIP is also newer which means it supports digital features that aren’t possible with PRI.

SIP trunks deliver voice and data services to companies over the Internet. Businesses don’t need a dedicated PBX to use SIP as providers offer hosted VoIP services. But if you have an IP-PBX already, you can use SIP for your voice communication.


Call Quality

Many people argue that SIP cannot match PRI when it comes to quality. There are reasons for this. PRI is not subject to network issues like jitter, lag, and packet loss. Historically PRI has delivered exceptional audio quality.

But this doesn’t mean SIP cannot match it. In fact, with the right set of tools and processes, SIP can also deliver exceptional quality. However, since it’s new, not many businesses know how to optimize for it.


Similar to quality, PRI is touted to be better since it is a physical connection. Generally speaking, it’s true. As long as nothing happens to the hardware, calls will reliably connect. But there are risks to copper lines as well. They can get damaged by weather like snow or rain. Copper lines also degrade over time.

SIP can also be reliable, provided you have fast internet connections and a good service provider. SIP lines don’t suffer from weather conditions. They cannot be damaged deliberately or accidentally. You can even use mobile failover options to make calls when the data connection is down.


PRI can be really secure, even though there are risks like interference. Calls can also be intercepted. Since calls don’t ever leave the private and closed phone network, it’s difficult for unauthorized users to breach security.

Being a network technology, SIP is subject to the usual security risks. Hackers can listen in on conversations, record calls, or even commit fraud. Yet again, SIP can be made secure with the appropriate tools.

Sending unencrypted data with consumer-grade SIP services over the public internet is a recipe for disaster! But with hardened firewalls, SBCs, VPNs, and encryption, SIP systems are very secure as well.

Cost & Maintenance

If you shop around, you may be able to get similar rates for PRI and SIP trunks. But in most markets, you’ll get far better rates for SIP. Since there’s very little physical hardware, there’s no extra cost to buy or maintain.

If something happens to your PRI line, you’ve to wait for a technician to come over and repair it. There’s virtually no physical maintenance with SIP trunks, reducing costs even more. Over the long term, SIP gives you more savings as you add extras like messaging, video conferencing etc.


This is one of the areas where SIP is the clear winner. Remember that a PRI line comes with 23 voice channels. So you can have 23 simultaneous conversations within the company. What if you need more, say 27? Well, you have to buy another PRI line that gives you 46 channels. You pay for an extra line even though you only need 4 extra channels.

PRI isn’t very flexible. It takes time to scale. Suppose you’re in the middle of your annual contract and fired a dozen employees. You don’t need 2 PRI lines anymore. Too bad, you’re stuck paying for them until the end of the year!

SIP providers allow businesses to add SIP trunks with an online request. If you use hosted VoIP, you may need to change the subscription plan to allow for more users. If your business downsizes for some reason, you can remove lines as well. You don’t have to wait for a contract to expire or even the end of the month!

Which one is better for your business boils down to what you have and need. If you have in house experts and don’t have access to high-speed internet, then PRI is suitable. But for most companies, SIP will end up as the best option for scale, flexibility, features, and not to forget, cost!