[Asterisk-Users] diffrent area codes for diffrent phones in dialplan

[Jer]

I have 3 sets of SIP phones all in diff area codes that need to access the PSTN

I need to it so that a 7 digit number is converted to a 10 digit with the 
correct ara code

eg a call coming from sip-phone1 needs aera code AAA and a call coming fom 
sip-phone2 needs BBB
how can this be setup in the dialplan
is there someway to set a var on a per sip group basis?
I thought of the accountcode...since i will not be using it for CDR


thoughts

Thanks