Losing another Customer

This a complaint not against MTN Meyerton branch but to MTN Contract department. I have been with MTN for a long time and got my first contract in 2012. I have upgraded ever since and also had a few other devices on MTN contract. So what I want to say is, I have been a loyal customer for a long time. I upgraded my contract on the 1st of February 2018 to a Sony Xperia XA 1 Ultra. After about 2 weeks, it had no signal at all. I returned it to MTN Meyerton, and they had been very helpful by sending the phone in for repairs as per MTN policy. A week later MTN Meyerton phoned me and advised me that I can collect a new device. I collected the new Sony Xperia XA 1 Ultra. This was now the second phone that had a lot of updates which used MY data. When I got my FIRST call on this phone, the other party could not hear me. I have tried many different numbers and even restarted the phone and still the other party could not hear me. I returned the phone to MTN Meyerton and asked for a different kind of phone as this batch of Sony might give me more problems and I do not have time to run to the shop every time my NEW phone does not work. I have a business to run with this phone. The lady at the counter spoke with head office and I was told I can get a different phone with my package as long as the device retail value does not exceed R5108. I do understand that I have to pay in when I take a more expensive phone, however the Sony Xperia XA 1 Ultra retail value in MTN book is R6899. The person at MTN Meyerton explained to me that the contract is based on the package I get and the device is for free. If that is the case, why am I forced to take a cheaper phone with the same package? It does not make any sense to me. I have decided after 6 Years of loyalty to MTN I want my contract canceled.