Hang on. All this complaining. Could it be the user and not the phone? This FAQ might be a solution:https://zentalk.asus.com/en/discussion/7175/low-in-call-volume-on-zenfone-6-one-tip-to-easily-solve-this-problem#latestIf you hold the phone to your ear differently, maybe you don't need to raise the volume as high and there won't be distortion issues. Could it really be this simple? Ear piece location, look at the FAQ.
Actually most people probably don't know about the position of the speaker. More volume to that earpiece, the worse the quality. Seems to simple I know. Most people won't admit they might be wrong about an issue and that the fix is nothing more than holding the phone in a different position. Hold the phone differently means less volume required which means higher quality audio. Too simple.
I think the call quality has improved after updating to latest version WW_16.1220.1906.167_0 ... Will still see for the few days but currently i think it is acceptable...
We would like to know which country and area you're all from? This will help us locate the issue faster.