Page 153 - Asterisk™: The Future of Telephony

The s Extension
Because of the technology we are using in our channels, we need to cover one more
thing before we get started with our dialplan. We need to explain extension s. When
calls enter a context without a specific destination extension (for example, a ringing
FXO line), they are passed to the s extension. (The s stands for “start,” as this is where
a call will start if no extension information was passed with the call.)
Since this is exactly what we need for our dialplan, let’s begin to fill in the pieces. We
will be performing three actions on the call (answer it, play a sound file, and hang it
up), so our extension called s will need three priorities. We’ll place the three priorities
below [incoming], because we have decided that all incoming calls should start in this
context. ‡
[incoming]
exten => s,1,application()
exten => s,n,application()
exten => s,n,application()
Now all we need to do is fill in the applications, and we’ve created our first dialplan.

Note that we could have numbered each priority as shown below, but
this is no longer the preferred method, as it makes it harder to make
changes to the dialplan at a later time:
[incoming]
exten => s,1,application()
exten => s,2,application()
exten => s,3,application()
The Answer(), Playback(), and Hangup() Applications

If we’re going to answer the call, play a sound file, and then hang up, we’d better learn
how to do just that. The Answer() application is used to answer a channel that is ringing.
This does the initial setup for the channel that receives the incoming call. (A few ap-
plications don’t require that you answer the channel first, but properly answering the
channel before performing any other actions is a very good habit.) As we mentioned
earlier, Answer() takes no arguments.
The Playback() application is used for playing a previously recorded sound file over a
channel. When using the Playback() application, input from the user is simply ignored.

‡ There is nothing special about any context name. We could have named this context
[stuff_that_comes_in], and as long as that was the context assigned in the channel definition in sip.conf,
iax.conf, zaptel.conf, et al., the channel would enter the dialplan in that context. Having said that, it is strongly
recommended that you give your contexts names that help you to understand their purpose. Some good
context names might include [incoming], [local_calls], [long_distance], [sip_telephones], [user_services],
[experimental], [remote_locations], and so forth. Always remember that a context determines how a channel
enters the dialplan, so name accordingly.

A Simple Dialplan | 125