Restrictions passcode problem

I am trying to reset my daughters old phone to factory settings but it keeps asking for a ‘restrictions passcode’. Neither me or my daughter remember setting this up so have no idea what it could be (many failed attempts).

whats the best way to be able to reset her old phone. We have tried to reset all settings but it asks for the ‘restrictions passcode’, we have tried erasing all content and settings but again it asks for ‘restrictions passcode’. We seem to be stuck. Any other suggestions would be very much appreciated.

Her phone is an IPhone 6 Plus version 12.5.5