Mtn poor service

I am extremely annoyed and disappointed wih service and lack of interest by mtn. I am a medical doctor who is on call and i have been telling the store at which i got my new samsung s9 about the new device dropping calls since i recieved it in april (within one week)the sales person at the mtn branch will atest to this. In the interest of time and my profession not allowing me to be without a phone for long period of time mtn advised me to book it at the service centre which i did last week. It was returned to me after upgrading software and i was told it was fine i now have been using the device for the last week and still it drops calls even while on the phone with the salesperson at the mtn durban north branch in hyper by the sea it dropped the call several times. During the period it was booked in i borrowed a 4 year old htc which worked fine, so tell me how is this no a issue with this device. What was the point of upgrading my samsung s7 to a s9 if only for this new device to be faulty makes no sense and it is infuriating that mtn even though my phone is in the warranty period mtn cant simply replace my device. Im not asking for the world im simply saying that under terms of my contract within 6months of warranty i have the right to have it replaced. Considering this is one of my 3 mtn contracts i expect far better from a large company like mtn. Disgusting.