Internet Protocol (IP) has revolutionized networking for the enterprise. Whether it is data or multimedia (voice/video), IP networking offers benefits that legacy systems cannot match. According to Transparency Market Research, the SIP market will be valued near $35.5B by 2030.

With SIP-based telephony offering multiple technical and economic benefits over PRI-based systems, it is certainly time to make the PRI to SIP conversion. Gartner says “U.S. enterprises of all sizes should migrate to SIP trunking, which will reduce monthly telecommunication expenses by up to 50%”. At CarrierBid, we’ve seen this savings first hand and highly recommend PRI to SIP conversions for all of our customers! But first, let’s get a little background about these modern communication technologies.


Primary Rate Interface (PRI) refers to the traditional method of providing large business enterprises with telephony services. The service works through an on-premise digital Private Branch Exchange (PBX) connected to the service provider over T1 lines. A single PRI can handle 23 voice calls simultaneously: additional PRI circuits need to be added to accommodate increased calling requirements. Making calls requires the use of a physical device.

Session Initiation Protocol (SIP) is a protocol that allows for transmission of voice over a data network. Two options are available for SIP telephony:

  • Hosted PBX: all hardware is hosted at the service provider, and the customer is connected to the service provider network over high-speed Internet or MPLS. Benefits include low up-front cost and scalability.
  • On-premise PBX: a physical SIP server is installed at the customer premises, and is linked to the service provider network over high-speed Internet or MPLS.

Any SIP-enabled device – IP phones, computers, laptops, smartphones etc. – can be used for making calls. It is very easy to scale up: additional licenses and a bandwidth upgrade are all that may be required for increasing the number of concurrent SIP calls.

PRI to SIP Migration

Now that we have a little background of PRI and SIP, let’s discuss some of the primary considerations for PRI to SIP conversion. One of the key aspects to consider is the benefits of using SIP compared to PRI, such as enhanced flexibility, lower costs, and the ability to use a wide range of devices for communication. Understanding these advantages will help in making an informed decision during the conversion process.

1.  Interoperability

SIP is an IP-based service and each carrier has its own implementation. Therefore, it is important to ensure SIP interoperability, especially before moving from one carrier to another.

2.  Hosted PBX

Hosted PBX

Most enterprises will be required to swap hardware as they migrate from PRI to SIP. Instead of opting for the expensive on-premises PBX, you can go for the hosted PBX aka cloud-based option. This can help you lower the PBX implementation costs as you migrate to SIP and increase your capability to scale up and add more phone connections as you grow.

3.  Network Ownership

Having knowledge and understanding about the architecture of the carrier network that you will be using can be extremely helpful as you perform the PRI to SIP migration. You can ask the carrier about their built-in resiliency and interconnect options. This will help to ensure that you receive more reliable service.

It is also important to know whether the carrier network is capable of integrating with different PBX systems. Some carriers may limit your options if you are not allowed to use specific hardware. Increased flexibility will automatically result in increased control for PRI to SIP conversion.

4.  Caller ID and Switches

As you make the change to SIP and change carriers, it is essential to make sure that each and every network entity has received your updated information. Even if your service provider has all the correct information, calls will not be routed correctly if other network entities have not updated their switching database information.

Likewise, caller ID details can be misplaced while switching carriers, preventing clients from seeing your information when you call them using your new setup. Here, it is important to take advantage of Local Number Portability to ensure that your phone number remains the same even when you switch carriers.

5.  Functionality


The modern phone communication service is very different from traditional analog services. It brings with it a host of new functionalities increasing efficiency and offering improved and reliable services. SIP brings excellent convergence features ensuring that any network-connected device can be used as a softphone. It allows users to make and receive calls from computer systems that have the correct software installed.

Hosted PBX platforms also allow business users to send instant messages that can interact with different platforms. Furthermore, you can generate reports, metrics, and analytics to enhance your current business processes and communicate with increased flexibility.

6.  Session Border Controller (SBC)

The Session Border Controller or SBC functions as a gateway for call routing. It is like a digitized switchboard operator making sure that the calls reach their designated destinations. SBC’s also enable security, intelligent session control, interoperability, scalability, and the selection of the proper codec for incoming and outgoing calls.

When organizations migrate to SIP, the SBC can be used to set parameters to allow or reject international calls etc. Most carriers offer integrated SBC capabilities, which facilitates the PRI to SIP conversion.

7.  Emergency Access

When you migrate from PRI to SIP,  you must make sure that emergency calls, like calls to 911, are getting routed correctly.

8.  Conference Calling

Unlike PRI, SIP is not limited to 23 conference call participants. You can conveniently conduct high-quality video conference calls with as many participants as you need. This feature is extremely useful for large enterprises with decentralized office locations: users can join in from any location as long as they have access to a softphone or an IP phone that is SIP-enabled.

9. Fax

While TDM PRI’s handle faxes quite well, we really don’t recommend using SIP for inbound faxing even though there is a T.38 protocol to enable it. We’ve found that due to TDM to IP conversions across various networks, it usually has a LOT of problems. Conversely, outbound SIP for most of our healthcare customers seems to work well.


SIP-based voice communication is the way of the future. In this article we have considered a number of factors that will help you to plan a successful migration from your current PRI-based system to a modern SIP-based system.

The team at Carrierbid can help you understand how SIP fits into your specific environment and business needs. We can perform detailed assessments and assist you in developing the solution that is right for you. Our experts can be reached directly at by filling out the form at the bottom of the page or by calling us toll-free.

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