On Call and Underpaid in IT/IS?

An Anonymous Coward asks: "I work for a Fortune 100 company, that I wish to remain anonymous. But recently an issue has come up dealing with on call pay; we are responsible for monitoring systems outside of normal business hours, but are hourly employees. It has been brought up to management if only being paid when an issue occurs is legal, since we must be ready at any moment to respond to system troubles/pages outside of the normal business hours we are paid for, which includes not being able to go out of town, etc. I've asked around, but with little success, being that a large portion of Slashdot readers are in the IT/IS field I would think that someone has dealt with simular issues and would be able to offer suggestions or outline their on call pay structure so that so that I can approach management/HR. Any examples of past experiences or how you dealt with such issues would be helpful, management is more than willing to work with us."