Undisclosed terms or don't know how your own systems work or should work??

I purchased a cellphone from Truworths in August 2017. According to Truworths, this is a 6 months interest bearing transaction which I understand to mean payment including interest is to be paid over 6 months from date of purchase. Funny enough, there is still an active balance on the account up to today when I have paid every installment on my Truworths account up to February 2018 which is when the cellphone should have been paid off. I have been instore where a Truworths employee assisted me by calling head office to enquire about this and was merely told I was paying the interest. This response is NOT ACCEPTABLE. 6 months means 6 months. Nowhere has Truworths disclosed that interest would be paid after the 6 months or how long it would take to pay this off. Also, what about all the interest that this balance is accumulating now, out of the 6 month period? Why am I liable to pay this? Additionally, this is not the first time Truworths have done this on my account. I purchased a cellphone from them early in 2016 and found that there was still a balance on the account late into the 2017 year (about one and a half years later). After calling head office numerous times I was given no solution and it was only when I settled my account in full that this balance decreased. Please Truworths, I am losing faith in you and I see this as rather ********* behaviour.