Unauthorised access to my account

I would like to know how is a person has access to my account. I received an SMS from someone who was listening to a call between me and my sister. this person has access to hearing all our conversations and then sends us SMS's about our conversation. My sister and I currently are contract subscribes on MTN and are constantly being threatened by this person and grave invasion of my privacy. this is a risk of my profile being viewed and accessible to a third party. I am dissapionted to know that this is happening with all the *****s that could possible take place with such access.