Contract cancellation

I requested on the 21st of February 2016 for my data contract to be cancelled as the end of my contract was the 11th of March 2016.<br> in March I received a bill for data only.<br> In April my bill was way more than the amount I normally pay per month and I don't understand how and why my contract was not cancelled within their turn around time.<br> I contacted the call centre on the 8th of April and was promised my account will be sorted and my number changed to prepaid within 72 hours and that has not happened.