Switching to VoIP

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Chapter 17: Asterisk Reference

Asterisk pattern matching is relatively tame compared to regular

expressions, which is supported by other soft-switch systems like SIP

Express Router. But if you really need complex pattern matching with

Asterisk, you can pass data as arguments to the Asterisk Gateway

Interface (AGI), where you can use regexp in Perl or Python pro-

grams, and then pass it back into the dial-plan.

With the flexibility of Asterisk’s application commands, variables, priorities, and

contexts, there are nearly infinite ways of combining contexts and extension defini-

tions in the dial-plan.

Special extensions

Special extensions are alphabetic characters that have a logical meaning in the dial-

plan. They are used to describe scenarios in which an extension number doesn’t

exist, as when a call first enters the context from the outside world.

A—Asterisk

This is where calls go if callers press the star key during a voice mail greeting.

i—Invalid

This is where calls end up if the user has dialed an invalid DTMF or if the dial-

plan has a bug that transfers a call to a non-existent extension.

s—Start

The de facto extension that handles all incoming calls on channels that don’t

specify any other extension when first connecting.

h—Hangup

This is the extension you can program to handle ...