The United States Department of Labor defines an on-call employee as someone who must remain either on the employer's premises or very close to the site while not actually working and the employee can't otherwise effectively use that time for his own purposes. However, on-call status isn't always clear-cut, so whether or not the employee must be paid depends on the specific situation.
Factors for Consideration
Federal law states that you must be paid for hours worked, so the more limited your activities are during your on-call time, the more likely you are entitled to compensation. Time spent waiting for employment-related activities, such as the arrival of a certain customer at the premises, means you should receive payment for that period. If your employer does not permit you to consume alcohol or leave your house during an on-call period because you might be called in to work on short notice, that's more of a gray area. While you experience certain limitations, you also are free to engage in personal activities. If you must simply leave contact information with your employer in case you are required to work after regular hours, you're not considered "on call."