MTN contract is a ****

I had contract with MTN for 3 years it was end February 2025 I call them to cancel it end of February after 2 weeks I received email saying I must pay 700 for dual deal I call them to ask for the recording where I was canceling my contract they told me they will get bck to me which they didn't. How can they cancel 1 phone because it was a dual deal and want me to pay 700 while I was paying 530 For the past 36 months I never missed a payment but now when I'm done they trying to rob me this is not good