Billed incorrectly

In November I was billed incorrectly, I have been charged for the use of internet and for the international calls made. I have always been under top up option how could I have exceeded my limit??. The reason for this error could be because I was sharing the 073 with someone or is it because there are two accounts under my name of which I do not understand how it happened even with the new number that I have been given I am still sharing it with someone that was conformed by one of the consultants who assisted me in Menlyn (Aubrey). The invoice for November was never emailed to me? could it be because they knew I was de*****ed??