iPhone 6s asks for an old phone number

My little brother has a phone and asked me to help him but i am lost. Its a 6s and he wants to reset it because i am buying him a new phone for his birthday. He forgot his password and when we go to reset it, it ask for a phone number that was phone number a long time ago. We don't remember the number and neither does our mother. how can we reset the password or the phone, if we don't know the number?

[Edited by Moderator]