The real question is whether you are properly classified as a salary exempt employee, and I would be happy to analye that for you. But a quick answer is that yes a salary exempt person can be required to take on extra duties. Of course, there should be some kind of trade off such as comp time, but not required.
"On Call" is one of the worst drawbacks to being "salaried". I remember many restless nights from when I was a prosecutor, no comp time. So, I understand the frustration.
If you want me to go over your employment status, I'll need:
1. Approximate salary level. Is it above 75K?
2. What is your normal shift or length of duty on a given day?
3. Are you a licensed professional, nurse, lawyer, etc?
4. Can you describe your duties, management with hiring/firing resp., supervisor over a line, random work sites, etc.
5. How many hours do you work in a week?