Charges to account

Again the second time MTN called me and told me because iam such a good customer and so long the give me free data and so on i asked the lady more then 3 time are you sure you not going to charge me no its for free for the remaining of you contract... and guess what. Today i got my statment and they charge me again. This happend in February also. MTN or you can credit me and cancel all my contracts as you are lying and this is not you keeping by contact or your word. How can you treat customers like this and keep lying?