Connection / Faulty Device or Sim?

We had 2 Samsung S6 Edges bought on a contract with Vodacom. The 1 phone had a screen repair done and with this Samsung informed me that I will have to replace the main board. This was proven to be a defect in the device as it was heating up extremely from the date of purchase. After the Samsung service centre had the phone for nearly a month and a week. We collected the phone again. The phone appeared to be in good order. After about a week of use the phone calls that I made and received was met with enormous complaints of ****led sound or sounding to be underwater. After attempting the speaker phone I noticed that the caller could not hear me at all. Tried the Bluetooth in the car and was that even then they can't hear me. I took it back to Samsung and they said it must be the LTE network. But the other s6 same network does not give the same error even in the same area. Vodacom says that it is the device and not the network. Neither of the two institutions are willing to put it in writing and it always ends up costing me time to travel and petrol plus loss of clients as they can't hear me speak. Both parties blame the other. 2 of the same phones same network 1 with problem all ar