Your phones require a reliable and consistent internet connection for calls. If you often have calls where the sound breaks up or sounds bad, there is most likely a problem with the internet connection. You should download and run the Speedtest.net app on your computer, and test the connection. If it's possible to do this while on a problem call, that may be most informative. Often the internet connection quality will vary throughout the day. You can send us the results of the test for evaluation, but generally you want to be sure that the speed is above 10 Mbps, latency is below 50, and jitter is below 10. Below is an example of a great test result, from our own office.
You can download the software here, or run the test right in your web browser: https://www.speedtest.net/
From our end, we are not able to make any changes that improve call quality. This is always related to internal (LAN) or internet service issues.