Bad service

I am always surprised how Cell C is still in business. I have not pleasant experience with them. Recently they sent me an email saying that i owe them an amount of R288.67.I cannot say why because i make my payments on time and i have a contract with them which i am paying for (R99.00 5 gig anytime and 5 Gig night time. My problem is that it is not the first time,