Telkom do not honour said amount for contracts.

I have taken out a contact with Telkom for R305.00 a month. I have sign a debit order for the amount but they taken what ever amount they want and always on a different date. Example of this: On the 2 March 2021 they deducted R305,00. On the 31 March 2021 they deducted R507,70. Last night they wanted to deducted an amount but I am not sure how much it was because I didn't have money in my account yet.