Indefinite wait for faulty iPhone 6 replacement.

I checked my faulty iPhone 6 (8 months old) into the MTN HVRC in Durban on Tue 15 Mar. On Wed 16th was told it is a manuf problem and the unit would be swapped out. BUT... there's no stock, so have to wait. No ETA. Complained like mad that an indefinite wait is being asked of me - but my billing continues - and yet iPhone 6's are as common a phone as you can get. After some escalations, was told that stock will arrive on Wed 23rd and I'll get my replacement then. Today is that day and still no stock. Still being asked to wait indefinitely. No other options available. Have been told that the Dbn HVRC will call me to discuss, but am still waiting for that call. There are so many things that drive off that iPhone (insurance tracker, apple wallet with expiring vouchers, Whatsapp threads) that to be without it indefinitely is not an option. I expect MTN has to be able to give me a date/replacement phone or release me from my contract so I can take out a new one (there are iPhone 6's in-store but I'm told I can't have one of those!!!!) It is unacceptable to ask me to wait indefinitely, keep billing me and not give me options. The fault on the phone is not my doing!!