dishonest business practice and failure to refund money paid for non-existence service

In February 2016 I visited MTN repair centre (Midrand) to have my phone fixed. later on the following month I received a notification message from MTN stating that my phone was ready for collection, for which I went with a view to collect it. however, before the cell-phone equipment could be handed over to me I was told to pay the amount of R5,811.31 for the seemingly job performed. surprisingly, when the cell-phone equipment wrapped in the brown box handed to me, the cell-phone was not the one I brought into the repair centre, but the new cell-phone which they independently without my consent chosen for me which was part of the agreement/contract bargain in case they fail to fix the phone . I immediately requested to the speak to senior person in the store to explain to me the reason for such disingenuous dishonest business practice which ill treat and undermine customer freedom of choice. reason being that if I wanted a new phone I would have decided on which one I wanted, not for MTN to unreasonably impose such decision on me. another, the phone that I brought for fixing was more expensive in value than the one they presented. the manager at that repair centre promised to refund the money paid and to return the device (email available to confirm it). However, since February 2016 until to date, I haven't receive either my money or cell-phone, or a phone call from MTN despite having made several attempt to get clearity.