SIP Trunk

A SIP trunk (Session Initiation Protocol) is a packet based transport signaling session connecting SIP enabled devices to Internet Telephony Service Providers (ITSP’s). SIP trunks are internet based signaling sessions which open voice and video streaming paths across your internet lines from a local phone system to the SIP Trunk provider. SIP Trunk providers then route the call to local or long-distance telephone networks.

PRI Trunk

PRI (Primary Rate Interface) consist of 24 circuit switched digital channels running at the speed of 1.544Mb/s they are often referred to as a T-1 circuit. Each of the 24 channels is considered a DS0 and provides the path for a telephone call. Calls flow from your local business phone system to a local telephone company. The telephone company then routes the calls to a local or long-distance phone.


Like any internet or phone connection PRI and SIP trunking both require a physical connection into your business. PRI requires a Data Terminal Equipment (DTE) connection, which is normally delivered via a T-1 circuits. T1 circuits are four wire telephone circuits. The T-1 line connects to the DTE, which is normally a small box which is located in the phone closet or basement of the building. The DTE then connects via a cable to ether a router, channel bank or the business phone system. SIP Trunking can utilize the same style of connection, with more flexibility.

SIP Trunks are virtual packet connections. SIP terminates over any internet connection which has available bandwidth. Because of this flexibility, SIP offers many more options and lower cost of termination. If your business has a cable modem, un-channelized T-1 or other data connection, SIP Trunks offer easy connections.


PRI T1 Trunks give you 23 available channels for voice calls. If you have 23 channels today and then need more, typically you require another PRI T-1 line and 23 channels. This becomes very expensive to scale due to the increments of 23 channels. Fractional T1 PRI lines are offered. This would allow you to scale in smaller increments with PRI, but in reality the cost model for this rarely works out in your favor. SIP Trunks on the other hand are IP packet based so scalability is easy, providing you have enough bandwidth in your system.


When reviewing the cost, hardware is one major factor. PRI requires a T1 termination and T1 line are normally in the hundreds of dollars monthly. The T1 hardware includes the DTE and special connection devices on your router or phone system. These include T1 cards and T1 PRI router interfaces. SIP Trunks do not require special hardware. Since they are virtual IP packet connections they connect via any internet connection into your office. SIP Trunks are delivered to your office via the same internet connection your computer system uses.


SIP offers your business connection flexibility and ease of use. While PRI trunking is an older technology, it offers a high level of reliability. We do not recommend PRI. The technology is outdated and is only needed if your Key System or PBX Systems require PRI. SIP providers have very competitive rates, ZOBOLT will provide you a quote for SIP Trunks. SIP Trunk cost can be as little as $20.00 per month. PRI trunks cost in the hundreds or thousands per month. For midsize business over 200 employees, using SIP trunks connected to their on-site phone system can save thousands yearly. While we still recommend Hosted PBX for small offices, below 200 users. SIP Trunking is a great way to save money for any business.

About Author: I am a communications systems specialist and Army veteran. My experience includes, switching systems, and consulting. I’m certified on class 5 switching elements and Cisco, Extreme routing network equipment. I’ve worked as a network engineer for over 20 years.


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