I work as a crisis counselor and therapist at a center for at-risk youth. In addition to the hours I'm scheduled to spend at the center, I spend two weekend days per month on call. While I'm on call, I have to carry my phone at all times. I can't go out of town or even out on a long run, as I have to be able to make it into the center within 30 minutes of a call. I can't drink alcohol, either. While my employer pays me for this on-call time, it's at a lower hourly rate than my other hours at the center. Is that legal?
Yes, it is legal for your employer to pay you at a different rate for on-call time and regular work time. If your on-call time qualifies as "work," you are entitled to be paid at least the minimum wage for it. However, there's no legal requirement that your employer pay you the same amount you earn when doing other types of work.
Typically, the controversy over on-call time is whether it counts as work time at all. To answer this question, the law looks at whether you have the right to spend the time as you see fit or whether your employer has control over your time. The more restrictions placed on your time, and the more likely you are to have to actually work during your on-call time, the more likely you are entitled to be paid.
It sounds like your employer has already resolved this issue in your favor, though. So, as long as you are getting paid at least the minimum wage in your state (or, if higher, the minimum wage in your city) for your on-call time, your employer is acting legally. Of course, this doesn't mean you can't try to advocate for a higher wage. But that's a topic for negotiation between you and your employer, not a legal matter to be resolved in the courts.