Can't cancel mtn contract!!

Why it is so difficult to cancel an expired cell phone contract with MTN? I have been trying to cancel my expired contract for the past two weeks with no success. When you call 135 they transfer you to the Retentions/Termination Department. The first message you get from the Retentions Department is about high volume of calls. You hold for more than 30 minutes until the call drops. You go to the nearest MTN outlets, they can’t assist but direct you to call 135. I sent a complaint to their complaints department and they promised to come back within 24 hours. Its now a week and i have not received any response from the Complaint Department. I don’t know whether I must now lodge a complaint against the Complaints Department. Are they doing this deliberately to keep on charging me?