Service was not good

I went to mtn (boitekong branch) as it is difficult for me to reach their toll free number but the person who helped me could not come with solutions to my query,she told me that it is the same because even if she can help me mtn will debit the money from my account i ask her to borrow me they phone as to contact their customer service but she could not help.So if i want to cancel my contract as i finish paying and i dont want them to debit money in my account where should i go because it is the same place where i started to do my contract.