[start]
exten => _X.,1,do some stuff
exten => _X.,2,do some more stuff
exten => _1xxx5551234,3,goto(callout,s,1)

[callout]
exten => s,1,do some stuff
exten => s,2,do some more stuff

include => extensions

[extensions]

; define the extensions here

include => noextension

[noextension]
exten => _X.,1,what to do if not an extension