Union or no union, makes no difference. Federal law requires that employees who are hourly or non-exempt salaried be paid for all hours they work for the benefit of their employer as long as the employer knows or should have known that they are doing it. Being on-call may not necessarily count as work time, but if the employer restricts where you may go or what you may do, the time is likely to count as work time and must be compensated. They can tell you that you are on call, but they can't call you without paying you. Likewise, they can't tell you that you have to be at any particular place at any particular time without paying you.
An exempt employee is also on the clock for those times, but there is no requirement to pay them any more because of it. They get the same pay every week if they work 15 minutes or 100 hours.