Is being on call acceptable without pay?

Hello,
I work for a very large company as a part time sales associates (there are no full time sales associates). The shifts comprise of for the most part one manger and 2-3 sales associates on the floor. Managers are on salary while all part time sales associates are hourly and no commission is earned on sales.
I have a question about their scheduling process. For example, mgmt schedules me for three days of on call, which means for those days, I have to call into the store manager an hour before the shift starts to see if they want me to work. If they do, I basically have an hour to get to work, if they do not want me to work, then I don't go and I am not paid. It feels like my life is on hold because every day I have to wait and see if they want me or not..is this procedure legal? Many of us are fed up and wonder the same thing. This is practiced company wide but I don't recall seeing it in the employee handbook. I would appreciate your feedback. Thank you