Back in the dark ages when 'Monitor' wasn't working, and 'Standby' wasn't working, a number of ATCOs including HD and me used the unofficial 'Listen' eg. "Listen one one eight decimal five, Good-day"
One simple, two-syllable, easy to pronounce word, one meaning, hardly likely to be confused with anything else, and generally very effective…
Only a few were daft enough to call up and say 'hey, Tower, I'm listening'…
('No you're not, you're talking…')