[Freeswitch-users] gateway selection by prefix

I have 3 gateways: Telecom Italia on an analogue adapter MessageNet (sip provider) Skype How can I select gateway by a prefix THAT THEN GETS DROPPED before Dialing.. Example I would like to have All calls beginning with 9 Sent to MessageNet , DROPPING THE "9" All calls beginning with 7 sent to Skype, dropping the "7" Otherwise use telecom