Three digit is going to be a much bigger pain than it is worth, too many numbering conflicts, your customer will have to live with it, just match the last three digits to what they have and say to them you just have to dial 2 plus the extension or 1 plus extension to call the other site.

www.myrandomviews
"Old phone guys never die, they just get locked in some closet with an old phone system and forgotten about"

Retired, taking photographs and hoping to fly one of my many kites.