You mean like it actually is today with mobile phones?

This may be how it is in the States, but in the UK all mobiles phones start with 07 (other area residential codes start 01, 02 etc).

I always found it weird that the US numbering system assumes that mobile phones are static. Must be some weird technical debt buried somewhere.

The first mobile phone I used only had coverage in a particular area. It didn’t work outside of town.