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This really depends on how "right away" it has to be. If you could be at dinner, and then get up and leave that dinner, then technically that isn't a requirement to be there "right away." When the government has required an employer to pay wages for "on call time" the employee has had to be required to stay at a specific location (like in a workplace building) or to be within 10 minutes of a specific location.
If your employer just requires that, once you get the call, you leave what you're doing and go in, that's not compensible time because you do have the freedom to go and do things....they just may be interrupted. Fair or not, courts do not find that to be so restrictive that it is to be paid time. Only the time you are engaged in work is payable.
Finally, reducing your hours later in a week to avoid having to pay overtime is legal.
I wish I could tell you differently here, but your facts give no clear indication of any legal violations.
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