имхо верное решение:
[general]

[tutorial]
;incoming call
;IP SIP phone numbers begin with 4
exten => _4XX,1,Dial(SIP/${EXTEN})

;inner call
;call to local analog PBX phones
exten => _XXX/404,1,Dial(SIP/499/${EXTEN})
exten => _XXX/410,1,Dial(SIP/498/${EXTEN})

;long call
;call to any outer number
exten => _X./404,1,Dial(SIP/499/9${EXTEN})
exten => _X./410,1,Dial(SIP/498/9${EXTEN})

почерпнуто из The Asterisk Handbook Chapter 4: The Asterisk Dialplan "routing by Caller ID"