Switching to VoIP

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Chapter 4: Circuit-Switched Telephony

Hunt Groups

There’s nothing more frustrating than a busy signal when you’re trying to order a

pizza on Super Bowl Sunday. That’s why most pizza establishments use hunt groups.

Hunt groups are groups of trunks that allow incoming phone calls to circumvent the

busy trunk and “roll” to the next available trunk in the group. That way, the pizza

place never misses a call, even when their phones are ringing off the hook.

Centrex

Centrex is POTS enhanced with business-grade telephony features like call confer-

encing, four-digit dialing, and per-call billing rather than per-minute billing. It was

designed to curb the need for small businesses to invest in PBX equipment in order

to get modern telephony features. A single Centrex customer can use many Centrex

lines, collectively called a Centrex group. Within the group, each line can be called

using four-digit dialing instead of the usual 7-digit dialing (i.e., the caller can omit

the prefix when placing calls within her Centrex group). Some other PBX-like fea-

tures include the ability to easily transfer calls between lines in the same group, or

enable and disable call forwarding for a given line by dialing a special sequence of

DTMF digits. Normally, users of Centrex have to dial an 8 or a 9 at the beginning ...