Cell C Overcharges for contract

<p>I have a data contract from Cell C since 2015. It's a 3 gig contract. I pay R159 every month, on last day of the month. On 30 July they charged me R728.85 ! When I called the call center, they told me I went over the limit. I am asking them how, I signed for 159 per month and I explained I don't want them to add anything if the 3 gig is finished. For the past 12 months I never paid more than 159. There are times when the data would finish on 24th in the month, I would be unable to access internet, until they load data on the 1sy. This is *******. Fix it please.</p>