Mine

exten=s,1,Playback(custom/please-enter)
exten=s,2,Playback(custom/the-call-forwarding)
exten=s,3,Playback(custom/telephone-number)
exten=s,4,Set(TIMEOUT(digit)=5)
exten=s,5,Set(TIMEOUT(response)=30)
exten=_X.,1,DBput(CFIM/${CALLERID}=${EXTEN})
exten=_X.,2,Playback(custom/call-forwarding)
exten=_X.,3,Playback(custom/has-been-set-to)
exten=_X.,4,SayDigits(${EXTEN})
exten _X.,5,Playback(custom/goodbye)
exten=_X.,6,Hangup