PBX's with PRI Trunks

PBXs are used with PRI lines for the following:

Call centers, to receive the telephone numbers of callers

Individual telephone users for call screening

One voice mail system to support multiple PBXs

Dial-up videoconferencing

Large call centers use PRI ISDN to receive the telephone number of the person calling. With ISDN, the telephone number is sent at the same time as the call. However, it is sent on the separate D, or signaling channel. This is significant because it enables the telephone system, the PBX, to treat the telephone number information differently than the call. It can send the telephone number to a database that matches the number to the customer account number. The data network sends the account number to the agent's terminal that the call is sent to. It saves agents time by eliminating the need to key in account numbers.

Many corporation use PRI ISDN for incoming voice traffic. They local telephone company sends the caller's name and phone number over the signaling channel. The telephone system captures the information and sends it to the display-equipped telephone (the phones that are in most offices). Employees can use ISDN to screen calls. Unanswered calls are forwarded automatically into voice mail.

PRI ISDN private lines that connect PBXs together enable one voice mail system to be shared between multiple sites. The D channel carries voice mail signals that identify mailbox numbers and instructions to turn message-waiting indicators on or off. It also enables broadcast lists to be made up of users at different sites.

Companies with multiple PRI trunks can share the 24th signaling channel among a group of PRI trunks via nonfacility-associated signaling (NFAS). For example, an organization with 6 PRI trunks might have 4 of them equipped with 24 channels for voice and data. Two of the 6 would have 23 channels for user data and one signaling channel each with NFAS to support all 6 PRI trunks. Having 2 PRI circuits with signaling provides a backup in case one signaling channel fails. PRI lines do not work without a signaling channel.