Hotel City just introduced a phone call feature adding a new level of interactivity to the game -- the player has to respond to a ringing phone icon that randomly appears at the bottom of the game's control panel.
When you get a call, click on it to pick up the phone, and read an occupant's complaint about their room's temperature. It's either going to be too hot or too cold. Your task, then, is to find the room and click on it to fix the temperature. Do that, and you get a few hundred coins for your trouble.