[Asterisk-Users] Interesting Dial-Plan Question

[Matt]

Hi,
When I setup a user, I give them an extension like 570xxxxxxx.   This
is fine and dandy while in one area code, but we've since gone to
other area codes.    I'd like the user's to retain the ability to dial
7 digits no matter what number they have.   Any thoughts on how to do
that?

EXAMPLE:  User has number 7175551212.  I want that when they dial
3235555 it dials 717-323-5555.        User has number 5705551212.  I
want that when they dial 3235555 it dials 570-323-5555.

I'm thinking I need to do some chopping, splitting, and variable
writing?    Am I correct in thinking that if I do all of this before
my current dialplan kicks in that I should be able to still use my
routing?

IE: User dials 3235555 and is from 717 area code.   If I transform the
number to be 7173235555 the dial-plan will now read it as if the user
punched it in that way and process it?