Breach of contract

I upgrade to a new contract a month ago. When the new month come and I need to recharge on my contract I didn't receive my full package. I phoned customer services every day for a week long and none of the employers could assist me. After they said they put a request in for my problem, I only found out today that there was no request at all. I demanded a cancellation on my contract because I'm not happy at all with MTN who is breaking the contract after every month I pay faithfully my account. Now the employers of MTN said I can cancel my contract but I need to pay the outstanding amount on my account. Which I feel is unfair for me to pay it full if MTN is breaking the contract. If you don't let me cancel my contract without paying any amount I will sue you. And this is not the first time I pay for my contract and don't receive the full package. I'm really fed up with this BAD SERVICE!!!!!