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SIP Trunking vs. PRI: What's the Difference?

This article was updated on November 21, 2022

To a lot of experts, the SIP trunks vs. PRI debate has long been settled. If you ask for their opinion, they'll tell you SIP trunking is the best choice in many cases. For businesses with existing phone branch exchange (PBX) or other supported hardware in place, SIP trunking's virtual connection to the public switched telephone network (PSTN) offers flexibility, scalability, and other advantages that the older PRI can't touch. Throw in the sheer number of businesses choosing or migrating to SIP trunking solutions, and it's easy to see why some experts feel that way.

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As with many business technology debates, however, the true answer to deciding on SIP or PRI may not be as clear as it initially seems. While a growing number of businesses have moved away from PRI, both solutions have advantages and drawbacks that need to be considered. Here's a look at SIP trunking, PRI, and a few situations in which both solutions excel.

What Is SIP?

To some degree, the endless chatter about SIP vs. PRI has gone on so long because it's an apples-to-oranges comparison. While both offer businesses roughly the same result — a connection from the business's internal phone systems to the larger PSTN — the ways they go about reaching this goal vary enough to warrant discussion and consideration.


SIP trunking represents a virtualized take on business telephony. It works over a data connection, meaning companies can transmit their calls and multimedia messages over a suitable internet connection. For companies looking to move away from a standard PBX, gateways and other specialized hardware can be used to keep much of the infrastructure in place. Otherwise, a company may choose newer, higher-tech softphones and other updated infrastructure points.

That last point is especially important when considering one of the biggest SIP benefits: multimedia communication. For companies keen on unified communications and collaboration tools, SIP trunking represents the easiest, most scalable path to services like digital voice and video conferencing alongside higher-tech tools. If your organization makes heavy use of services such as desktop sharing, you're probably already using SIP trunking.

Turning back to scalability, SIP trunking's virtualized nature gives it obvious advantages over PRI's comparatively restrictive architecture. Where PRI's allowed number of active calls is effectively "hardwired," a SIP trunk is largely limited by the number of lines a carrier offers and the amount of bandwidth available to the customer location.

The same differences make SIP trunking a better choice for companies dependent on international calls. In much the same way voice over internet protocol (VoIP) drastically cut the price of international calls over standard landline calling, SIP trunking makes outbound international calls far more affordable compared to PRI. Additionally, SIP trunking generally represents lower month-over-month costs, especially if the alternative would be multiple PRI connections.


Because SIP trunking relies on a data connection, businesses must have sufficient bandwidth to support all of their VoIP phones and equipment. If companies receive a large number of simultaneous calls, they may need additional bandwidth to support all of their calling needs. While SIP trunking is generally considered cost-effective in the long term, the initial setup costs might give some companies pause.

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What Is PRI?


Connectivity accounts for PRI's biggest strength. If call uptime and quality are of the absolute highest priority for you — as in, even one hiccup or dropped call can cause an office catastrophe — PRI's stability and tendency to come with consumer-friendly service level agreements (SLAs) means you may have less to worry about. In fields like healthcare, where call quality is a top-tier concern, that's certainly worth considering.


Some people consider PRI outdated. Most U.S.-based customers deploy PRI over a dedicated T1 line. The same rough idea applies in Europe, where PRI is most frequently deployed over a dedicated E1 connection. While T1 and E1 are still used in business settings and offer benefits over standard internet connectivity, there's little question that they've fallen by the wayside in an era where public internet offers faster speeds and lower costs. This can immediately cast suspicion on the value and longevity of PRI in certain IT circles.

Because PRI primarily uses the two connectivity types, the need for T1 or E1 is arguably this solution's greatest drawback. The respective connections offer a set number of channels for voice (24 for T1, 30 for E1) and only allow exactly that number of simultaneous voice calls. For example, if you're using PRI in the U.S. and your location needs to place 35 concurrent calls, you would require two physical PRI connections. This scenario will also leave 13 channels unused unless there's a rare circumstance that you need more active calls than your usual maximum number.

It's important to note here that active calls are not the same as active phone numbers. A company using PRI with direct inward dialing could effectively have hundreds of phone numbers but support only the preset number of active calls.

Head-to-Head Comparison (SIP vs. PRI)

If you're trying to make the call between SIP or PRI, it may be helpful to have a head-to-head comparison of their chief differences. 


SIP is ultimately more cost-effective than PRI in nearly all cases, especially at scale. SIP does come with some setup costs, but it also offers attractive advantages over when it comes to long-distance calling and special features.

Flexibility and Scalability

SIP is far more flexible and scalable than its PRI counterpart. Every time a company needs to expand its PRI capacity, it must order a circuit with 21 additional lines regardless of whether or not the full 21 lines are actually needed. Ordering a new PRI circuit can also take time and involve some business disruption. By contrast, a business using SIP trunking can scale service up or down as needed without committing to such a substantial ongoing investment.

Digital Transformation 

SIP trunking allows a company to gradually transition away from legacy business communications technology or fully modernize right away if desired. Businesses that take advantage of SIP trunking can access VoIP, unified communications, and other innovative technologies that make true digital transformation possible. The same cannot be said of a PRI circuit.

Business Continuity

SIP is attractive from a business continuity perspective, allowing you to preprogram redundancies in service in case a disruption or emergency occurs. You can use SIP trunking to reroute calls from one location to another, allowing your company to stay in touch with customers and business partners in the event of a technical failure or disaster. PRIs don't offer this same level of redundancy and availability during an interruption in service.

Which Is Best for You?

Ultimately, the SIP trunks vs. PRI debate depends on your business's communications needs and physical location. Smaller organizations in rural locations that may not have internet service with sufficient bandwidth for SIP trunking may find value in PRI. Meanwhile, larger enterprises in areas with better connectivity options may find SIP trunking a better choice. This is especially true if they require a large number of active calls and could make good use of advanced unified communications and collaboration features.

It's clear that SIP trunking represents more scalability, flexibility, and functionality than PRI. If these are factors you need from your communications system, the newer solution may be your best bet. If technical limitations hamper your location's connectivity options — and you don't see your company needing more advanced features because of or in spite of it — PRI may be for you.

Contact Vonage Business to learn how to modernize your business communications.

Vonage Staff

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