Is It Legal To Require Someone To Be On Call Without Pay?

Totally depends upon whether she is exempt under the Fair Labor Standards Act, the state she lives in, and whether she has a collective bargaining agreement. I have to carry a pager device and am not compensated for being available or responding to calls. I'm FLSA exempt. However, I don't get punished if they cannot reach me. A former boss tried to coerce us into being available. He's gone.