Bridge of Contract

I had a 24 months contract with MTN, i cancelled my contract on the 23rd month giving MTN a full 30 days to cancel my contract but the actually gave me the 25th airtime and data. After doing so they debited my account with a full contract agreed tarrif. On the 7th they actually taken all airtime and data that wasremaining in my account so what i did was to reverse the monthly debit cause i cannot pay for what i did not use. Now they send me an sms saying i owe them monthly tarrif with interest. You do not go arround bridging and then demand people to call you. MTN must call me and let me know how much i used of the 25th package they gave me and i will gladly pay them. I aint gonna pay for what i did not use period. And after this i should just port my number to a servise provider who can actually offer customers a better service, i have had enough of this ****a shop business.