For enterprises wanting to make full use of their installed IP-PBXs and not only communicate over IP within the enterprise, but also outside the enterprise a SIP trunk provided by an Internet Telephony Service Provider to connect to the traditional PSTN network is the solution. Unlike in traditional telephony, where bundles of physical wires were once delivered from the service provider to a business, a SIP trunk allows a company to replace these traditional fixed PSTN lines with PSTN connectivity via a SIP trunking service provider on the Internet.
SIP trunks can offer significant cost-savings for enterprises, eliminating the need for local PSTN gateways, costly ISDN BRIs (Basic Rate Interfaces) or PRIs (Primary Rate Interfaces).
There are three components necessary to successfully deploy SIP trunks: a PBX with a SIP-enabled trunk side, an enterprise edge device understanding SIP and an Internet telephony or SIP trunking service provider.
A POTS line, usually just called an analog phone line, can hook directly to a standard telephone. It can be one of several lines that you select with a key telephone system or it can connect directly to a PBX phone system.
A PRI (Primary Rate Interface) line is somewhat different and more popular than a standard Voice T1 because it is true "digital trunking", which allows for even more advanced calling features. A PRI line provides 23 telephone lines per T1, because the 24th channel is used to carry useful data for signaling and other features like special caller ID information called ANI (Automatic Number Identification). Since the transmission is digital, the quality of a telephone connection over a PRI line is unsurpassed, providing crystal clear reception that is noticeable to the user. The use of DID (Digital Inward Dial) numbers, and calling features like hunting and rolling, allow you to use 23 digital telephone lines to provide service to potentially 30 to 40 employees, depending on how often they will need to use their phone. Instead of guessing on the number of DID's to put on a PRI line, it's important to figure out the maximum number of simultaneous calls that could be placed at any given time. Having too few telephone lines causes costs more in a loss of productivity than it would cost to simply add more phone lines to keep your employees busy. This is why it is important to figure out the proper balance of telephone lines vs. DID numbers. The cost of a PRI is also typically lower than other forms of telephone service (except Voice Over IP in some applications), with actual pricing depending on the Provider of the service and your business location. Between the features, price, and quality of a PRI, it is the primary choice for businesses large enough to take advantage of its multiline functionality and benefits.