Unknown amount debited off our account

I signed a Data bundle Contract from Telkom for an amount of R149.00 per month.

On the 2nd January a debit order went off my account from Telkom for an amount of R649.00 Today I went to Telkom to query the amount and was told I have an add on Internet Bundle which they billed me for.

I find this strange as I have not signed a contract for such sort. If my bundle is fine I purchase data. Further to that I was away during December period and did not use my data usage.

I would like an investigation into this matter, an explanation as well as a refund.