No to MTN contract

I have a contract with Mtn, I paid for 24 months and I would like to cancel my contract, I am told that I owe R877 because I have been using free data for 3 days. This only happened because I didn't want to upgrade my contract. I have been a loyal customer to them for 24 months. I want to cancel my contract. My phone has been off service for 4 days now. I have missed important calls. In January I recharge with R110 airtime and bought 1 GB data and only received 480MB, I called to enquiry and was told they will investigate, until today I am still waiting for my feedback. I will never do contract with mtn again