I think it's the other way around: Since calls were routed geographically, and it's not apparent to the caller whether they're calling a landline or mobile phon

I think it's the other way around: Since calls were routed geographically, and it's not apparent to the caller whether they're calling a landline or mobile phone, it also made sense to price them geographically (at least before the marginal cost of the incremental call-mile went to zero).