PRI Vs SIP – What to Choose?

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Part 4/4 of the blog series on Session Initiation Protocol. Part 1 Part 2 Part 3

SIP trunking, in comparison to conventional PRI system, offers many benefits to companies. It enables businesses to replace traditional PSTN lines that have PSTN connectivity using a SIP trunking provider over the internet. The major benefits offered by SIP trunking include the elimination of local PSTN gateways, significant cost savings over PRI and also mitigate the need to install costly PRIs or ISDN BRIs. So what is the difference between PRI Vs SIP?

PRI Vs SIP – What To Choose?

PRI (Primary Rate Interface) is a voice technology that has been widely used since the 1980s. It is an interface standard used on an ISDN (Integrated Services Digital Network) to deliver multiple lines of voice and data into a business’s existing PBX via one physical line, called a circuit. PRI is a high-capacity service carried on T1 trunk lines between telco central offices and your location. Most businesses have their ISDN PRI circuits in the form of T1 or fractional T1 lines. A T1 line carries voice and data via 24 digital channels.

PRI is considered “old-school” telephony. It is physical hardware and also requires servicing from a telco for deploying, upgrading and troubleshooting.

SIP is a way to deliver voice via the Internet. SIP is a telephony networking protocol (much like other network protocols such as HTTP and SMTP), therefore it’s a network technology rather than a telephone technology like PRI.

SIP trunks are virtual; they don’t require additional hardware to deploy. A business can use SIP for voice without the need for an existing PBX since many SIP providers offer hosted PBX. As with PRI, SIP can deliver multiple voice lines to a single organization.

Below is the summary of major advantages SIP trunking provides over traditional PSTN circuits.

Scalability PRI Vs SIP

PRI: New circuits need to be installed and additional termination hardware is needed to achieve scalability. For PRI the minimum increment is 23 voice channels.
SIP: Scalability is easily and quickly achieved. A software configuration change can facilitate on-demand and automatic burst capabilities.

Hardware PRI Vs SIP

PRI connections are physical. Each circuit requires costly termination software and physical connections are required
SIP connections are virtual. The number of available trunks in a SIP connection is a function of bandwidth available rather than physical termination circuits or hardware.

Capacity Planning PRI Vs SIP

PRI: Capacity needs to be planned in advance given the fact that considerable lead time is required to order, install and commission new circuits and termination hardware.
SIP: Although capacity planning assumes significance, it can be achieved quickly and easily. A simple software change can facilitate higher capacity and providers also offer burst capabilities for brief periods when a sudden rise in utilization arises.


PRI: Cost is usually calculated per circuit per month irrespective of usage. PRI is inefficient if you need one or a few more voice channels than the fixed increment of 23 channels.
SIP: SIP billing is usually usage based along with the capability of on-demand capacity upscaling. In comparison to PRI circuits, IP trunking involves lesser costs.

Disaster Recovery and Continuity

PRI: While diverting calls to an alternate location is possible, the process can be increasingly complex and cumbersome and can entail significant costs.
SIP: SIP technology leverages the ability to reroute calls to a pre-defined location in the event of a disaster thereby ensuring no breaks in the call flow. SIP can also accommodate diversity across service providers with Internet access via BGP

Redundancy/ Backup PRI Vs SIP

PRI: Provision of backup circuits to remote locations in an IPT distributed architecture entails high costs and negatively impacts ROI.
SIP: SIP trunking facilitates automatic IP re-routing to enable practical distribution of PSTN connectivity to locations with limited or network redundancy.

Hybrid Trunking
Yes, there are use cases where you can mix SIP and PRI.

PRI:  for local calling and then uses a SIP trunk from a hosted VoIP service for international calls as a way to save money.
SIP: SIP trunks are increasingly replacing PRI, legacy PBX systems can use SIP trunking by implementing a VoIP gateway.

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