The control yoke microphone switch has two functions. When the top side is pressed it activates the "PTT" or push-to-talk for the selected communications radio. When the bottom is pressed, the flight interphone is active.
Crews do not like having to key the interphone to have normal conversation in the cockpit. It was not uncommon to see a rubber band wrapped around the switch to keep it engaged.
My airline has modified the yoke microphone switch to lock itself in the interphone position. A small ball rides up and over a mount extension.
A tool adjusts the height of the ball. To far up..... the switch won't lock. To far down..... and it blocks the switch from being used.
As a note: When the switch is locked in the interphone position, the cockpit speaker is muted out. It is assumed the both crew members have their headsets on for communication.