Does 3CXPhone auto gain the mic? In a few successful calls, sometimes the recipient cannot hear me loud enough. It's afterwards that I notice the mic level is set to low Windows internal value instead of "63".
The mic volume setting is directly linked to Windows's mic volume slider in your sound settings.
Increasing/decreasing mic from windows will directly affect your mic volume on the 3CX VoiP phone.
One way to find your optimal mic level is to launch Skype and Windows mic properties, enable "Automatically adjust microphone settings" in Skype, say a few things and observe the value/slider in mic properties. Jot it down and set the level after you quit Skype.