Billed even though there was a cap on my contract

I had converted my data contract to preapaid. Ref number :*** . This was effective in March and I was on prepaid from 01/04. I was still debited R149 which was the contract fee on 01/04 and then R288 on 04/05. I called the call centre to query the billed amounts as I had cancelled the contact and was on pre-paid. I was transferred from customer services , to accounts ,to the data department then back to accounts who told me this looks like a data charge who transferred me back to the data department only to be told they could not see anything as this was already converted to prepaid who transferred me back to accounts who then wanted me to go back to the data department when I stopped them they said log a dispute with customer services . They transferred me and the person that answered kept asking me which department I was trying to get through to. I do not understand how a contract that was capped manages to get to a value of R288 and how I get a bill when this was converted to prepaid. When I queried the capping of the contract I was told , \Read the terms and conditions and it doesn't always work"What is the point of a cap if you are not sure it will work !"""