[internal]

# The internal dialplan (forwarding someone to a registered phone)
exten   =>  _[1-8]XX,1,AGI(internal.php)
exten   =>  _[1-8]XX,n,GotoIf($[${SUCCES} = 0]?failure)
exten   =>  _[1-8]XX,n,GotoIf($[${SELF} = 0]?callingself)
exten   =>  _[1-8]XX,n,GotoIf($[${OUTGOING} = 1]?outgoing)
exten   =>  _[1-8]XX,n,Dial(${LOCATION},30,t)
exten   =>  _[1-8]XX,n,Voicemail(${EXTEN}@default,u)
exten   =>  _[1-8]XX,n,Hangup()
exten   =>  _[1-8]XX,n(callingself),Answer()
exten   =>  _[1-8]XX,n,Playback(the-number-u-dialed&is-in-use&by&your&telephone-number)
exten   =>  _[1-8]XX,n,Hangup()
exten   =>  _[1-8]XX,n(failure),Voicemail(${EXTEN}@default,u)
exten   =>  _[1-8]XX,n,Hangup()
exten   =>  _[1-8]XX,n,Goto(outgoing,${LOCATION},1)
exten   =>  _[1-8]XX,n,Hangup()

[outgoing]

exten   =>  _0ZXXXXXXXX,1,AGI(callerid.php)
exten   =>  _0ZXXXXXXXX,n,Dial(Zap/g1/${EXTEN})

[phones]
include => internal

# the internal.php AGI sets LOCATION to something matching _0ZXXXXXXXX if it sets outgoing to 1

Output in asterisk:

[Dec 29 17:29:41] NOTICE[25708]: pbx.c:1877 pbx_extension_helper: No such label 'outgoing' in extension '300' in context 'phones'