Sounds like you're needing to set up your PRI to use call-by-call signalling (part of the PRI protocols available) then build separate digit modification strings to insert the call flag digit (to indicate the call type) ahead of dialed digits, example:
caller dials 9-1-(area code + number)
Normally you would simply strip the leading digit 9 and send the 1 + 10 digits to follow to the LD carrier.
With CBC signalling (after you have enabled it on the PRI and reset the PRI) your mod digits may have to include an additional digit INSERT (dependent on what is required by the CBC definition) so you can tell the carrier what type of call this is (domestic or international or inter-LATA, etc)
Your big 3 carriers (AT&T, SPRINT, MCI) usually don't need this but often some of the competing carriers (ie., Broadwing) do.