Active contract number given away to someone else.

Mtn decided to give away my number to another customer even though I have an active contract with them with this number for four years! . I feel like my entire identity has been given away to some else as my life is linked to that number including my clients. It's a gross infringement of my privacy as this woman is now receiving all my messages. She even took over what was my whatsapp and changed my profile picture. I've been with mtn since I was a teenager but I'm not so sure about them. I'm very disappointed and heartbroken at all that has transpired.