Yes, your company is required to pay employees if you require them to remain on your premises while they wait for an assignment (for example, firefighters waiting for an emergency call). If this is the case, they are considered to be working and must be paid, even if they are doing other things, such as playing cards.
No, you don’t have to pay employees if you allow them to go home and they are free to leave messages saying where they can be reached. In most cases, like these, the employees are not considered to be working.
Yes, you must pay employees if you allow them to leave but restrict their activities, (such as requiring them to remain close to the workplace or not drink alcohol while on call).