Configure asterisk with wildcard extensions

This post covers a common use case when deploying a phone system: Somebody in the business needs to be able to record voicemail greetings for other people.

This assumes that you are already running asterisk, and that people already have something in your Dialplan (extensions.conf) for people to record a greeting:

; Record voicemail greeting
exten => *,1,AGI(scripts/record-voicemail-greeting.pl);

To run this script as somebody else, in the phone sense, we just need to change the CallerID before we run it. Some things you’ll need to know to do this:

1. An “X” in the extension matches any digit
2. EXTEN is a variable holding the current extension
3. CALLERID(num) is another variable, which holds the CallerID number
4. ${EXTEN:2} is a “substring”, which cuts the first two letters off the extension

With that in mind, if * records your own voicmail, then **4567 would record 4567’s voicemail using this snippet:

; Record other person's voicemail greeting
exten => **XXXX,1,Set(CALLERID(num)=${EXTEN:2})
exten => **XXXX,2,Goto(*,1)

Of course, it would be a terrible idea to enable this for the whole business, which is why you can also check the CallerID before you change it. This alternative snippet allows you to record any voicemail greeting, but only if you are calling from 1234.

; Record other person's voicemail greeting (if calling from phone 1234)
exten => 1234/**XXXX,1,Set(CALLERID(num)=${EXTEN:2})
exten => 1234/**XXXX,2,Goto(*,1)