If you have a forwarding phone number linked to your Google Voice number, and you have disabled forwarding to that number, then Google Voice has no idea whether that linked number still belongs to you. Many users link a forwarding phone number, and then abandon the number (switch carriers, discontinue service). After some months, that number gets re-issued by its carrier and then, your inbound GV calls get forwarded to somebody else's phone. To prevent this, Google Voice periodically asks you to verify your linked number(s) if they aren't being used. If you don't want to get these notes, then a) forward calls to that number, or b) delete the number off of your list of linked forwarding numbers.