I’m in the same situation. Calling and receiving calls worked for a year or so via the app, but at some point this year, it asked me to verify that my (former) US number was still valid (which I obviously couldn’t do). Probably what OP is referring to.
Guessing I’ll have to find another alternative soon.
Did you have the number configured as forwarding number? At some point I got asked to verify an old forwarding number, but I just deleted it and Google Voice continued to work fine (I mostly use it in VoIP mode instead of forwarding calls).
Guessing I’ll have to find another alternative soon.