T1 PRI Lines (aka ISDN PRI T1) are used by small and large business, call centers, fax servers, to provide more than just local and long distance phone service.   There are a number of reasons why PRI T1 lines are the preferred voice communications solution.   The main reason is the increased functionality and flexibilty it offers over regular phone lines, VOIP, or traditional Voice T1 phone service. 

A T1 PRI line is a digital phone service consisting of 23 B channels (bearer channels aka phone lines) and one D channel for signaling, control, and call data communication between a PBX and the phone company switch.  Because it is a digital phone service (not VOIP) the quality and reliability of service is unsurpassed and the functionality of service is enhanced through communication with D channel.  The ISDN PRI D channel will set-up and breakdown calls quicker, provide robust caller ID info which can be tied to a backend database (for screen pops with info on customer calling in), allow for the use of Direct Inward Dialing (aka DID), as well as other advanced call routing features.    ISDN PRI lines are also used for dial-up data services mainly for remote internet access, vpn, or ISDN video conferencing, creating an on demand data solution.  PRI T1 lines can also be bonded together to create one trunk group for mulitple PRI circuits (so calls rolls from one circuit to next), increasing capacity to handle more inbound calls at once without busy signals.   

As far as T1 PRI pricing goes, there is suprisingly very little difference in cost between a PRI circuit and VOIP PRI or Voice T1 phone service, see PRI pricing.   PRI T1 lines offer the best bang for the buck and great functionality when it comes to business class phone service.