Account Mismanaged

Hi, I have done a cellphone and data contract upgrade early this year which is capped, bt to my dismay every month my account bills r much more then the agreed contact. I have contacted MTN woodmead more then 5 times now and the store Manager Lucas, either hangs up the phone or made several promises that he will get back to me. What incompetence that a store Manager can't set up a contract? I then went to my nearest branch which is Lakeside Mall, there again the store Manager (Gloria Nene) tells me there is nothing they can do for me. I am very displeased and disgruntled by the service I have received from MTN thus far, that I even want this contract to b canceled at this point in time! How long can it take for someone to give me a decent service as a valued customer is beyond me!