Off Topic a Bit

Area codes, exchanges and extensions are US/Canada/Carribean concepts. Very few other areas use them. Germany for example uses over four thousand prefixes to represent local dialing zones, but they are not analagous to exchanges nor to area codes (and are of variable lengths). If you design your software with 3-3-4 in mind your software isnt going to work anywhere else in the world except maybe Switzerland (who use the NA system internally). My own telephone number is 11 digits for instance, and thats NOT including the country code. I know of peope who entire number is only 6 digits.

I would say that indexing based on the full CLI is the best, and most portable.