FNB return my cellphone

I have a cellphone contract with FNB, I took an iPhone 6 plus sometime last year. Within 6 months of using the device it died without warning, I then called their device centre to report it as it is still under warranty. The gentleman that assisted me informed that they would send a courier to collect it, that was on the 25/01/2018. Since then I have been calling to find out when I would receive the device back as I am still paying for it but no positive feedback. Last week I called on ********** again and the gentleman that assisted me informed me that they don't have the iPhone 6 plus in stock anymore so I would have to take the iPhone 7 but because it's more expensive I would obviously have a new quotation, when I asked about the current balance he informed that he wouldn't be able to give me an answer he would request someone else to call me within 2 hours and explain everything to me, a week later I still haven't received a call not have I received proper feedback but they continue deducting their instalment with a phone that they took back. Please help, I justl want a cellphone as I have been paying all these months without disputing their instalment, they must just please give the same respect and have integrity.

My name : Lelethu Xalisa Device REF : SDV ********** 3740 Contact no. ********** 605