The radios have a "do not disturb" mode you can turn on ie. when you have a layover. The ZEW buttons call all the radios, and unmute those in the DnD mode for the duration of one transmission. ZEW1 calls all of the driver radios, ZEW3 calls all of the dispatcher radios - they hear a beep and for example "Train on track 10 respond to Katowice" - then if you pick up the microphone the DnD mode turns off, and if you don't you won't hear anything until someone decides to push ZEW1 to call all drivers. The ZEW2 was used to call all the drivers driving in even direction, leaving ZEW1 for those heading odd direction, but it was discontinued a long time ago.
GoppelPL's post in What is the differences between ZEW (Dispatchers) 1, 2 and 3 ? was marked as the answer