I have a contract with MTN and I’ve had for almost a 2 years now hence it’s expiring next year July

I have a contract with MTN and I’ve had for almost a 2 years now hence it’s expiring next year July. My problem is which is once in a while they charge you extra on things you didn’t sign up, or rather I should say what they never explained to you maybe it’s because they know that you won’t take the contract. Recently I got a call from them saying that they’re realised that we as customers don’t use our night data,so they want to help us as they don’t want us to pay for something we don’t use. They gave us a chance to change that so that we can get chance to use it and migrate,the moment they said that I asked if there aren’t gonna be extra charges and they said that no there won’t be any. Now I’m surprised to see next month’s payments,it’s from R1 122 to R1 700 🚮just like that I’m overcharged,they are telling me about Prorata data of which I never asked for and it’s not the first time they are doing this.