MTN shocking service

This is my second really bad experience with the MTN store in Longbeach, Cape Town. My last upgrade was a nightmare, and my current one the same. I upgraded to a Huawei H12, which had problems. They replaced the handset with a new one of the same model. This one also had problems, which one of the store clerks took a video of (phone blackscreening and being totally unresponsive when making / receiving calls, and playing whatsapp voicenotes). As it was the 8th day, they said that they could not replace the handset, but would have to send it away. This is surely incorrect as the phone is faulty - not once but twice, and as a customer I have to come back and forth to the store, copying info back and forth... and now, I've been waiting almost two weeks for any kind of feedback. Nobody at the store responds to messages, MTN South Africa Facebook page simply ignores messages. I've been with MTN for 18 years, and this is unacceptable. All I am asking for is a working phone, and some kind of customer service.