Someone else made an Apple ID with grandfather’s phone number

My grandfather has a regular old flip-phone (not an iPhone, not even a smart phone). When I send him a message using iMessage, he does not get any message on his flip phone, and someone else responds saying “wrong number!”

I’ve verified that he CAN get messages via SMS though, if I disable iMessage, but it’s a really horrible workaround because I don’t want to have to disable it every time (or I will forget).

Any advice on how to fix it? Presumably the other person has an Apple ID that is tied to my grandpa’s phone number, and it is intercepting my iMessages.

How could they even use his number to create the account in the first place?