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. Please go to fast.com and run the speed/quality test there. 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.
Go to fast.com in any browser, let the test run, then click the "more info" button. You can send us the results of the test for evaluation, but generally what we are looking for is that latency is fairly low, and particularly, the loaded and unloaded latency are not very different, as seen here. The upload speed should also be good, as this is often where outbound voice issues come from (the person you called/called you hears problems).
Another good test to run is here: http://www.dslreports.com/speedtest
Select the type of connection, and let the test run. Then get the results, particularly the buffer bloat score. It is CRITICAL that you run this test while your computer is not otherwise busy, and that you let it run as the main tab in your browser without doing anything else while it runs.