Great question, unfortunately we don’t have much information on that (yet), so I looked it up and found some helpful material.
From the Guide to the Employment Standards Act “…an employee who is not at the workplace but is “on call” is not considered to be working unless the on-call employee is called into work.”
The ESA includes obligations on employers to pay certain on-call employees. Specifically, if an employee is required to be available for work, or is required to work, but ends up working for less than three hours, they have the right to receive wages for a minimum of three hours if their working time falls short of that duration. This provision for on-call pay is applicable only once within a 24-hour period, commencing from the start of the employee’s on-call duty, regardless of the number of times the employee is on-call.
Here is the full article if you want to read on: Employer Obligations to Pay On-Call Employees in Ontario
I hope this helps!