We'd be happy to look more into this and see. Can you give us more details on the type of problems you are seeing? Is it crackly, cut out, etc?
From there we can better diagnose the issue.
The first thing you need to know is if the phone is in WiFi Calling or cellular mode. If it's on WiFi Calling, it could be because of the quality of the WiFi/Internet connection. Disable WiFi Calling then try again. Try putting the phone into various modes: 3G, 2G (GSM), LTE and make some test calls. If it's bad in all modes, you could have a bad phone. If it's good in some and bad in some, there's maybe a network issue.