I wonder if the cellphone provider's topography/geography could be a factor?

In the US, I think it would be, yes. Dunno how it works in the UK.

For instance, imagine two cell phones with the same number, one based in the 530 area code, and one based in the neighboring 916 area code. The 916 number is owned by the video store.

Someone who lives in the 916 area code calls the video store without dialing the "916" area code, that's how they're supposed to do it. They do this for months, it works.

Then they decide they're tired of the awful service from their current cell provider and they buy a new phone at the mall which happens to be in the 530 area code. Or they move up the street and their land-line number changes to the 530 area code. Even if they understand this technically, they might not remember that they have to dial 916 to call the video store. So they get the schmoe in 530 instead.