Billed for carried over and new data

I had a contract with Nashua which was moved to Altech for the last few months until it completed. I cancelled the contract within the final month. I then started my final month and had data carried over and received my new data for the final month. I then completed the month with having data left and the contract expired. When I received my final bill, Altech charged me over R 1000 for data usage. I called and raised my complaint twice and until today nobody has come back to me. I am now being pressured to pay up my account. So I now need to pay a bill of data that was part of my contract and which carried over. Nobody could explain why I was charged for the data, bu it also seems that nobody is concerned as I am no longer a customer after cancelling my contract. I dont see why I must pay for the data that was part of my contract, just because I decided not to be a customer anymore, I will not pay this account as I cannot be billed for data that is part of my contract. Altech does not seem to care about assisting a previous customer, only about getting money