We have the basics of an auto-attendant created, but now let's make it a bit more robust. We need to be able to handle special situations, such as when the caller enters an invalid extension, or doesn't enter an extension at all. Asterisk has a set of special extensions for dealing with situations like there. They all are named with a single letter, so we recommend you don't create any other extensions named with a single letter. The most common special extensions include:
i: the 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.t: the reponse 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.s: the 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.
h: the 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.o: the 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.a: the 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.[demo-menu]exten => s,1,Answer(500) same => n(loop),Background(press-1&or&press-2) same => n,WaitExten()exten => 1,1,Playback(you-entered) same => n,SayNumber(1) same => n,Goto(s,loop)exten => 2,1,Playback(you-entered) same => n,SayNumber(2) same => n,Goto(s,loop)exten => i,1,Playback(option-is-invalid) same => n,Goto(s,loop)exten => t,1,Playback(are-you-still-there) same => n,Goto(s,loop)