******** deductions from my bank account

One more bad review for Cell C, sure they are use to it. I cancelled my contract on the 13th of May 2019, wereby I received sms confirmation that my contract will be terminated by 9th of June 2019. At the beginning of June they deducted a fraction of my monthly amount as per pro rata usage. That was all pretty fine. End of May, 2019, with cancellations in place, I move to the United Kingdom. Beginning of July 2019, they once again deduct the previous full monthly amount from my bank account. Now when I email them, they respond by saying they need to make telephonic contact with me for security reasons. How bold is that? They ********ly deduct money from my account for a cancelled month-to-month contract, then when I notify them that they need to investigate and pay my money back they respond by saying I need to be contacted for security reasons to confirm it is me? For their mistakes, really?! I gave them my UK cellphone number but haven't received a call yet.