Firefighters in the US typically work 24hr shifts, then have 48hr off, then repeat[1]. You could say they are 'on call' during that 24hr shift, of course they don't normally fight fires every minute of it (luckily), much of that time is spent doing things in the firehouse like maintenance, eating, etc.

1: https://firefighternow.com/firefighter-shift-schedules-and-w...