On Call compensation?

While clearly, being on call imposes some limitations on your life, such as that you have to remain within a distance to answer the call (so, no travel), you can still (we presume) do most things and maintain the majority of control over you time: you can sleep, socialize, shop, do errands or chores, spend family time, watch TV or read, go out to a restaurant, exercise or play sports, etc. Assuming that in the case, then you only have to be paid when aactually called in to work: the law accepts that being on call, whether as a doctor, a tow-truck operator, an IT tech, or other, imposes restrictions on your life, but does not consider those restrictions to be tantamount or equivalent to work--and so, not anything you need to be paid for--so long as you again, can, within your community or area, do most things that you'd want to do and can travel within that area (are not restricted to an office or building). So based on what you write, you would not need to be paid while on call. 
Remember: employment in this country is "employment at will." While discussions of that tend to center on the employer's power or ability to terminate employees at will, it equally means that employees can quit, resign, change jobs, etc. at will. Thus, the law allows your employer to put significant limitations on your life, since the idea is that if you don't like them, you can seek other employment.