To disable the DND function and remove the button from the softkeys on Yealink phones, enter the following override using the Device Defaults textbox (at the company, site, or department level), or using the Overrides textbox at the device level.  If using the Overrides textbox you will need to enter the override for each device.  Otherwise you can use the company, site, or department device defaults to change all phones within those groups.

features.dnd.allow="0"

After you've entered the override, be sure to Save and Resync the device(s) effected.