Special Asterisk Dialplan ExtensionsHere we'll list all of the special built-in dialplan extensions and their usage.
a: Assistant extensionThis 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 extensionThis 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 extensionWhen 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 extensionIf 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 extensionIf 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 extensionWhen 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.