Unfair billing

I have a contract with vodacom,this week I received a statement which I was charged extra 300, I contacted them and they inform me I had requested extra 250 minutes in December and January, I asked them when as I have never requested extra minutes and the voice contract is used by my son who doesnt even know how to request the minutes,I was told to request you only dialed *111# I asked how can one do that as the number is a contract and not prepaid and how can it be as simple as dialling those numbers, the lady didn't provide me with the answer and said I must pay cause I asked for free minutes.