contract cancellation

I cancelled a Telkom Contract in January 2016. Should be paying R299.00 for an upgraded phone only. Was charged R1505.00 compared to the normal +-R578 charged before the cancellation and upgraded phone. Telkom cannot explain this and 5 months later the contract is still not cancelled and I am still being billed. Is there anyone that can cancel a Telkom contract out there can assist? Unfortunately cannot get any assistance. Why must I be billed for Telkom's incompetency? I want all charges reversed immediately!!!