Special Asterisk Dialplan Extensions
Here we'll list all of the special built-in dialplan extensions and their usage.
a: Assistant extension
This extension is similar to the o extension, only it gets triggered when the caller presses the asterisk (*) key while recording a voice mail message. This is typically used to reach an assistant.
e: Exception Catchall extension
This extension will substitute as a catchall for any of the 'i', 't', or 'T' extensions, if any of them do not exist and catching the error in a single routine is desired. The function EXCEPTION may be used to query the type of exception or the location where it occurred.
h: Hangup extension
When a call is hung up, Asterisk executes the h extension in the current context. This is typically used for some sort of clean-up after a call has been completed.
i: Invalid entry extension
If Asterisk can't find an extension in the current context that matches the digits dialed during the Background() or WaitExten() applications, it will send the call to the i extension. You can then handle the call however you see fit.
o: Operator extension
If a caller presses the zero key on their phone keypad while recording a voice mail message, and the o extension exists, the caller will be redirected to the o extension. This is typically used so that the caller can press zero to reach an operator.
s: Start extension
When an analog call comes into Asterisk, the call is sent to the s extension. The s extension is also used in macros.
Please note that the s extension is not a catch-all extension. It's simply the location that analog calls and macros begin. In our example above, it simply makes a convenient extension to use that can't be easily dialed from the Background() and WaitExten() applications.
t: Response timeout extension
When the caller waits too long before entering a response to the Background() or WaitExten() applications, and there are no more priorities in the current extension, the call is sent to the t extension.
T: Absolute timeout extension
This is the extension that is executed when the 'absolute' timeout is reached. See "core show function TIMEOUT" for more information on setting timeouts.