Disable Call Waiting Tone Yealink T4X series
On your device press the "Menu" softkey.
Navigate with the devices arrows to "Features" and hit the physical "Ok" button.
Navigate to "Call Waiting" and hit the physical "Ok" button. 
Navigate to "Play Tone" 
Use the physical left and right buttons to toggle feature to "Disabled" and hit "Save"
Call waiting tones should now be disabled.