I'm being debited for a new contract which I am not liable for

After battling for more than a month last year (March 2018) to get an early upgrade from Vodacom I decided to take out a new contract, to my surprise 2 cellphones arrived by courier. One early upgrade and one new contract. I kept the upgrade and returned the new contract phone. December 2018 I noticed Vodacom started debiting my bank account with much more than the normal amount I usually pay, upon investigation it was found that the phone I returned was checked in as received on Vodacom's side but the contract was never cancelled and suddenly month's later they decided to start debiting me for this. I never opened the box the cellphone came in or the sim card and one day I decided to phone the number appearing on my statement and the number rang, I found this very strange. I am paying for a number which is not being used by me! Vodacom has now debited my bank account with this amount for 2 months and I have been fighting for 2 months to get it corrected and the money Vodacom now owes me refunded to me with no joy at all. One person will just direct me to the next just to hear that nothing has been changed on their system and they will do it now and the next time I call nothing still changed and after numerous promises that they will get back to me no one does. I received my latest statement and see that my next debit will be incorrect again. I have been a loyal customer to Vodacom for many years and I am now beyond frustrated.