Mtn giving my number to someone else.

I am very disappointed with mtn. I have been using my mtn number for a very long time and still use it for whatsapp but i have been getting different people telling me of and when they call the number another man answers the phone. People I am doing business with have been complaining. I wonder does this man received my whatsapp messages too since he answers my calls. This is so unprofessional in so many ways I am leaving mtn. I am not happy.