We make Push-To-Mute handsets (with the button built in to the handset) for thousands of phone models. Most people prefer having the button built into the handset, but if we can't make one for your phone, this is a viable alternative.
Connects in-line between the phone and the handset. Press the Momentary Push-Button on the box to disable the transmitter on your handset. When the button is pressed, the transmit circuit is physically disconnected.