Page 151 - Asterisk™: The Future of Telephony

Don’t worry if you don’t understand what Answer() and Hangup() are—we’ll cover
them shortly. The key point to remember here is that for a particular extension, Asterisk
follows the priorities in order.

Unnumbered priorities
In older releases of Asterisk, the numbering of priorities caused a lot of problems.
Imagine having an extension that had 15 priorities, and then needing to add something
at step 2. All of the subsequent priorities would have to be manually renumbered.
Asterisk does not handle missing steps or misnumbered priorities, and debugging these
types of errors was pointless and frustrating.
Beginning with version 1.2, Asterisk addressed this problem. It introduced the use of
the n priority, which stands for “next.” Each time Asterisk encounters a priority named
n, it takes the number of the previous priority and adds 1. This makes it easier to make
changes to your dialplan, as you don’t have to keep renumbering all your steps. For
example, your dialplan might look something like this:
exten => 123,1,Answer()
exten => 123,n,do something
exten => 123,n,do something else
exten => 123,n,do one last thing
exten => 123,n,Hangup()
Internally, Asterisk will calculate the next priority number every time it encounters an
†
n. You should note, however, that you must always specify priority number 1. If you
accidentally put an n instead of 1 for the first priority, you’ll find that the extension will
not be available.

Priority labels
Starting with Asterisk version 1.2 and higher, common practice is to assign text labels
to priorities. This is to ensure that you can refer to a priority by something other than
its number, which probably isn’t known, given that dialplans now generally use un-
numbered priorities. To assign a text label to a priority, simply add the label inside
parentheses after the priority, like this:

exten => 123,n(label),application()
A very common mistake when writing labels is to insert a comma be-
tween the n and the (, like this:
exten => 123,n,(label),application() ;<-- THIS IS NOT GOING TO WORK
This mistake will break that part of your dialplan, and you will get an
error that the application cannot be found.

† Asterisk permits simple arithmetic within the priority, such as n+200 or the priority s (for same), but their
usage is considered to be an advanced topic. Please note that extension s and priority s are two distinct
concepts.

Dialplan Syntax | 123