Apple ID

My grandmother has recently got a new phone (a Samsung) however I tried to text her back the other day and it went as an iMessage so I FaceTimed the number and an old man answered. But if I send a text message or phone the number it will go to my grandmother. So an old man is using my grandmothers new number for his Apple ID and we don’t really know what to do about it. We have contacted the phone company she is with and they have said they have said the number is new so someone else should not be using it so it must be something to do with Apple.