Q: Call fails randomly on iPhone 6 Plus

Hello, Apple Community. My fiancé and I got an iPhone 6 Plus from T-Mobile 2 weeks ago. Since day 1, she's had an issue with hers to where she can no longer hear the other person on the other end but they can hear her. This happens literately every call. And sometimes, it will have the call fail randomly with no warning. Previous to this we had an iPhone 5s and never, ever had this happen. We also used wi-fi calling with the 5s and 6. Nothing has changed from the locations we used the phones in and the connection. Just the devices. Has anyone else had this issue before? If so, what steps should I take to try to fix this? We already updated her phone and reset network settings. Any help is appreciated.