If one is exempt (salaried and not eligible for overtime) they are compensated regardless the number of hours worked, that does not mean a supervisor can't give them another day off if they have extraordinary time they are working for a project, but we don't comp hour for hour, more of a flexible agreed arrangement.
If the employee is non-exempt (eligible for overtime) they are compensated for all hours worked and unless otherwise written in a CBA, they earn overtime at time and one-half for comp or OT over 40 hours worked in a week. "On call" describes when an employee has limitations on how far they can be away from work or if they are handling work from home but expected to respond, the time should be compensable. Below, you'll find some guidance.
Can an employee still be on-call or performing work at home during a furlough day?
Whether on-call time is hours worked under the FLSA depends upon the particular circumstances. Generally, the facts may show that the employee was engaged to wait (which is work time) or the facts may show that the employee was waiting to be engaged (which is not work time).
For example, a secretary who reads a book while waiting for dictation or a fireman who plays checkers while waiting for an alarm is working during such periods of inactivity. These employees have been "engaged to wait." An employee who is required to remain on call on the employer's premises is working while "on call." An employee who is allowed to leave a message where he/she can be reached is not working (in most cases) while on call. Additional constraints on the employee's freedom could require this time to be compensated.