Unauthorised debit order

Due to financial constraints I fell behind on my Vodacom Data Account and last month the 25th of March a man claiming to be from Vodacom Collections phoned and we agreed to a debit order of R500 a month to pay off my due amount. The data line has already been suspended. On the 1st of April 2019 Vodacom deducted an amount of R1502-00 from my bank account. I can get no feedback and trying to phone the collections is a useless excercise on its own. I would like to know who gave them permission to withdraw such an amount out of my account. I have had absolutely no luck with customer care either, service is poor and very frustrating to say the least.