If you have someone on speaker phone and want to temporarily suspend that caller from hearing what is happening in your office, press the “MUTE/DEL” key in the bottom right corner. Then press it again to resume your 2-way conversation with the caller.
This mute function is useful if you need to have a private, brief conversation with someone in your office. 
The "DEL" function part of the key is for DELETE. It is most commonly used while dialing. If you press the wrong number in a dialing sequence, rather than hanging up and starting over, press “MUTE/DEL” to remove the incorrect number and then resume entering on the dial pad.
