MTN contracts a ****

MTN is trying to pull up a fast one with me. I had a contract with them where I was paying 430 for three years and now it has ended but they are saying I owe them 1750. I just spent the past 30 mins being transferred from pillar to post trying to lodge a dispute. I have heard people talk about how they will be billed even after the contract has ended and didn’t believe it. I wanna take MTN to the ombudsman. I can’t be ****med like this