Still charged for cancelled contract

I am utterly disgusted with Mondo Mobile and MTN.

I subscribed to a new MTN contract with Mondo Mobile in October 2017, and cancelled it in February 2018 because a. the two sim cards I received took MTN three months to activate, b. MTN refused to port my number because the contract was not entered into with MTN and c. I wanted the voice and data to be on one contract but was given two contracts and two sim cards.

After notifying Mondo several times of my intention to cancel I returned the handset and the sim cards to MTN in Greenstone, and was told that the cancellation was in order.

Now, as I was applying for a new contract with MTN I am informed that this contract is still in place and I am still being charged for it, and there is a huge amount outstanding!

What part of "cancelled" don't you understand? Mondo's cancellations department informed me that the cancellation was in order!

MTN says it's Mondo's responsibility and Mondo says it's MTN's responsibility. Why does that not surprise me...

I want this contract and charges removed from my name, or compensation of equal value, or I shall take legal action.