No phone broken phone

A few years back I bought my son a phone. So I could keep in contact with him without going through her. Over time she has hidden it, turned it off, said its lost ...... Now her own phone is broken. She puts 100s in the bank each month not short of money, To save her self £100 she has take his phone and now uses it as her own, Im now calling 20 times per day to try to chat to my son. You know what , the law lets her do it.