Relevant US case law is Berry v. County of Sonoma, 30 F.3d 1174 (9th Cir.1994) [1], holding that county coroners' on-call time (requiring carrying pagers and responding by telephone within 15 minutes) was not compensable under the Fair Labor Standards Act.
Two key factors are "(1) the degree to which the employee is free to engage in personal activities; and (2) the agreements between the parties." Beyond these general factors, no universal rule applies since the details matter (frequency of calls, response-time requirements and geographical limitations, etc) in the degree to which they limit personal activities, as do any agreements laid out in a contract or company policy (e.g., how specific 'on-call' requirements and compensation are defined and agreed upon in advance).
Note that these factors have only to do with the time spent _waiting_ for a call; time spent _actually working_ while responding to a call is more clearly work that should be compensated.
Two key factors are "(1) the degree to which the employee is free to engage in personal activities; and (2) the agreements between the parties." Beyond these general factors, no universal rule applies since the details matter (frequency of calls, response-time requirements and geographical limitations, etc) in the degree to which they limit personal activities, as do any agreements laid out in a contract or company policy (e.g., how specific 'on-call' requirements and compensation are defined and agreed upon in advance).
Note that these factors have only to do with the time spent _waiting_ for a call; time spent _actually working_ while responding to a call is more clearly work that should be compensated.
[1] https://casetext.com/case/berry-v-county-of-sonoma