My grandmother’s line is associated with another Apple ID that belongs to another relative but nobody seems to know who’s. Is there any way I can dissociate her number with the previous account. Nobody knows what ID has the number applied

I come from a large family. Recently, my dad got a used iPhone 7 for my grandmother because her iPhone 4 was getting buggy. She wasn’t logged in to an Apple ID on her phone. I set her up with an Apple ID to save her contacts and other items to iCloud. Now, when I try adding her number to the new Apple ID on the new phone, I’m getting a message that the number is tied to another Apple ID. Problem is, she doesn’t know what ID and nobody else does either. So I don’t even know who to talk to in order to have them remove it from their ID and given none of them are very techie, even if one of them did know if it was tied to the ID, they wouldn’t have the first clue on how to proceed removing it. Many of them have a difficult time even figuring out how to use the mobile browser. I have full access to her old phone and number which still works (surprisingly for such an ‘ancient’ phone by 2019 standards), but there doesn’t seem to be a way to look up the Apple ID by the phone number…or is there? And what should I tell said family member if I can figure this out? They all pretty much live in rural areas with no nearby Apple Stores or Genius Bars. So I’m their best shot at getting this resolved.