Unauthorised debit on Edgars account

My mom's Edgars account keeps getting debited with Cell C airtime, although she has never used Cell C nor has she authorised this debit on her account. She has been into the branch multiple times but the issue has not been resolved. Now she is getting SMS's threatening legal action if she does not pay the account. She tried calling the number on the sms and keeps getting put through to different people and getting told to email different addresses etc. I would like this resolved ASAP. Please cancel this debit order and credit my mother's account.