There are specific guidelines to follow when paying an employee to be on-call. The rule of thumb is that if being on-call restricts or limits the employee from having the freedom to come and go, the they must be compensated for the time they are on-call.
For example, if the employer requires the employee to be within 30 minutes from the office in the event they are called, then their free time is affected and they should be compensated. However, if the employee is free to go wherever and just needs to answer a cell phone or respond to an email, they are not limited with their free time, then they would only be paid during the call or while responding to an email.
If you have more questions, I suggest consulting with an experienced HR professional.
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