Switching to VoIP

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Chapter 12: PSTN Trunks

Dial-Tone Trunks

Choosing from the plethora of legacy voice connectivity technologies isn’t a no-

brainer. When you choose a dial-tone trunk solution to supply your voice switch with

a path to the outside world, you should consider the capacity of the solution, the

implications for quality of service, geographic availability, and, of course, the cost.

POTS and Centrex Trunks

Plain old telephone service, introduced in Chapter 1, is an analog phone line from

the CO that connects to your PBX using one copper pair. This is the kind of phone

service you most likely have in your home. The POTS line is cost effective when

fewer than 10 lines are concentrated in one location. Most POTS lines are paid for

monthly, with overage charges added when a certain level of utilization (minutes)

has been surpassed. POTS lines are available just about anywhere there’s electricity

and running water. Each POTS lines can support one phone call at a time.

Centrex phone lines are POTS lines with special, business-related calling features like

four-digit private endpoint dialing. These are the lines on which you have to “dial a 9

to get out.” Centrex has the same economics as POTS—you generally pay a monthly

fee that covers a certain amount of utilization on the line; after that,