A zip code doesn't guarantee that you live inside city limits. I would like to think that Verizon has been around long enough that they get the boundaries directly from the city's GIS department.

The reason that's important is that various municipalities tax things like telecommunications, especially for franchise fees, 911 fees, etc. So the telcos need to know who to pay.

I have a family member whose ZIP Code and government boundary are all consistent, but Verizon serves them via a line from a bordering town so her phone number and infrastructure is all attached to that other town.

This stuff gets complicated even without ZIP Codes involved.