Sim card hacking

My sim card stopped working, no network but because MTN Network is forever horrible, i overlooked it. Only to find out that my sim card was hacked into and the hackers got access to my communicating with them pretending to be me asking for monies which Im now owing. How can such a big company let such things happen to their clients. It's frustrating finding out from their agents that these things happen all the time and there's nothing we can do about it. I'm owing money because my sim was hacked.