contract changed & reactivated without my consent

*** - contract was sold to me in Jan 2015, device (tablet) was delivered to myself & upon seeing what I was offered I immediately returned the device as I felt it was not going to be of any use to me, I called after a day or so to confirm if the device was received & it was confirmed that it was but to my surprise a couple of months later I was still being debited for the same device that I had returned & the contract was never cancelled. After numerous attempts to cancel this contract I received a sms confirming that I will be credited back the money that was debited from my acc since January 2015 which was R95.00 per month, according to my calculation R95.00 X 6months =R570.00 but the sms that I received stated that I will be credited R488.11 so please explain to me what happened to the rest of the money & why was this contract not cancelled immediately when the device was received back from me. I do not know where the credit note was credited to & when<br>