Well anyone can get a phone number just by typing it in. All it takes is using a fake (not your own, belongs to someone else) number. Or typing in the wrong phone number (typo) when trying to access an account. Things like that. So it doesn't mean it was someone having stolen your number. They can't actually be using your phone or a phone with your number. It is more likely what I just mentioned. It is no different than someone calling you looking for someone else because they dialed wrong or were giving the wrong number.