Depends on what the operator asks of you.
"Hey, just a heads up, we might have a need for a pilot on this date, wondered if you have anything scheduled yet." = No charge but also take first trip that offers money.
"Hey, I have a trip on this day but it might cancel. I need you to be available if you're interested." = Daily rate, I bet the salaried pilot gets a paycheck if it cancels because he is garuntees his availability.