Extensiones Predefinidas en Asterisk

Predefined Extension Names

Asterisk uses some extension names for special purposes:

• i : Invalid
• s : Start
• h : Hangup
• t : Timeout
• T : AbsoluteTimeout
• a : Asterisk extension
• o : Operator

Special extensions

Because all the programming logic must occur via extensions, we need some additional system-defined extensions.

The h is the standard "hang-up" extension. The h extension, if it is configured, is called when a caller hangs up the phone. Note that as soon as this happens, the content of ${EXTEN} changes to h.
Example

Say we want the global variable CONNECTIONS to reflect the number of currently active conversations at any given time. This means we need the value of CONNECTIONS to increase by one every time a connection is initiated and decrease by one every time someone hangs up. The following dialplan illustrates the basic idea:

[global]
CONNECTIONS=0

[from-internal]
exten => _X.,1,Set(CONNECTIONS=$[${CONNECTIONS} + 1]|g)
exten => _X.,2,Dial(SIP/${EXTEN})

exten => h,1,Set(CONNECTIONS=$[${CONNECTIONS} - 1]|g)

The i extension

To make a context gracefully handle every conceivable circumstance, we use the special extension i ("i" stands for invalid) which handles all dialed numbers which are not explicitly handled handled within the context. Again, as with the h extension, once the i extension is invoked the ${EXTEN} will no longer contain the dialed number. To get the dialed number while in the i extension, use ${INVALID_EXTEN}.
Example

In our example business, Widgets, Inc., employees in department B can only dial extensions 100 to 199. Callers dialing any other numbers hear the message, "I'm sorry. That is not a valid extension. Please try again."

[department-b]
exten => _1XX,1,Dial(${EXTEN})

exten => i,1,NoOp(An invalid number ${INVALID_EXTEN} was dialed.)
exten => i,2,Answer()
exten => i,3,Playback(invalid)
exten => i,4,Hangup()

The o and a extensions

If operator=yes is set in voicemail.conf, the call will be directed to the o extension (o is for operator, kids!) if the caller presses "0".

Pressing * (asterisk) will direct the call to the a extension (abort).
The t and T extensions

The t and T extensions are for handling timeouts in the context.
t extension

If there is no input in an IVR menu within a certain time, the t extension is called.

Example:

[mainmenu]
exten => 10,1,Answer()
exten => 10,n,Background(marryme)             ; "Marry me? Press 1 for y
es, 2 for no."

exten => 1,1,Playback(thank-you-cooperation)  ; 1 => "Thank you."
exten => 1,n,Hangup()

exten => 2,1,Playback(hangup-try-again)       ; 2 => "Hang up and try ag
ain."
exten => 2,n,Hangup()

exten => t,1,Hangup()                         ; no input => hang up

T extension

The T extension is called after the absolute timeout has been exceeded. You can set this timeout value with Set(TIMEOUT(absolute)=<seconds>).
[Warning]    

Be careful not to have any spaces before and after the "=" character.

The timer starts whenever the timeout value is set (in other words, it does not automatically start with the connection, it must be started explicitly with the Set() command). Set(TIMEOUT(absolute)=0) deactivates the absolute timeout.

Example:

exten => 20,1,Answer()
exten => 20,2,Set(TIMEOUT(absolute)=120)
exten => 20,3,Playback(hello-world)
exten => 20,4,Wait(1)
exten => 20,5,Goto(3)

exten => T,1,Wait(1)
exten => T,2,Playback(thank-you-for-calling)
exten => T,3,Wait(1)
exten => T,4,Hangup()

The s extension

The first entry in any extension is always the name or number dialed by the caller. When a call comes in from the PSTN, however, Asterisk doesn't know what was dialed or whom the caller is trying to reach. For any scenario in which we cannot determine the number dialed, we use the s extension.
[Warning]    

If you are using an ATA device (analog telephone adapter) you don't need the s extension. You can configured the dialled number in the configuration interface (often a web interface) for the ATA.

Example:

exten => s,1,Answer()
exten => s,2,Wait(1)
exten => s,3,Play(tt-monkeys)
exten => s,4,Wait(1)
exten => s,5,Hangup()

http://www.the-asterisk-book.com/unstable/besondere-extensions.html