Overbilled for data usage

On 01/11/2015 I was billed R711 for data usage on my contract. I then went into the N1 City branch to query this and dealt with a gentleman by the name of Lance Skippers who was very helpful. He showed me how the billing occurred, however my query was why I was billed for this as I had 3 data bundles still active. When these data bundles are depleted, I would then get a automatic sms advising me of this. Therefore when I did get this sms, there was at least 1 data bundle still active so at no point did I not have data on available on the data bundle. However I was billed additional for this data even though I had data bundles active and did not receive a sms comfirming the last data bundle was depleted. Lance Skippers then escalatedthis to your call centre who provided the reference number***. Two days later I received a sms confirming that the data usage is valid. Now I know that I have used data, my query is why was there additional billing for data when I had at leats one data bundle active? And if this was depleted, why was I not notified of this as per the Cell C agreed contract? I then called Lance Skippers on 03/11/2015 and left a message - no response