debiting acount without permision

hi. i have a contract with Vodacom. haven't renewd or cancalled it yet. yeserday they debited my account with R646 without my permission. i called them this morning regarding this. they told me that i upgraded the contract wich I never did. i never sighned or agreed to anything. they told me they looking into the matter and will go listen to a voice recording. they never got back to me. the only thing I ever told them that if i decide to upgrade I will go to the store personally.