Labor law varies from state to state. You'll have to consult an attorney or some other person knowledgable about the law in your particular state to get a trustworthy answer to your question.
But perhaps you could ask someone in your Human Resources department whether or not they are aware of any laws, regulations, or guidelines related to "on-call" status. Your question may stimulate them to look into the situation -- and their review might lead to something good for you.
I don't see why it wouldn't be legal. I get $2/h for on call (plus min. 2h pay if I get called in), but at least in my state employers are NOT required to pay for on call time, as long as the employee is able to pursue their own activities during call time. If you have to stay in house, they are required to pay you a normal hourly wage, as that is considered working.
All the RNs at my job are required to take call.