Is There any Standards or Legislation for On Call Pay?

I am wondering if there is any labour rules or legislation that relate to employees who must be on call for weekends such as sales staff. I know that there is a mimum call-in pay of 3 hours for work performed outside of regular hours for production staff, but is there a requirement to pay employees who are required to be “at the disposal of the employer” such as a sales employee who is on call during the weekend and must be ready to come into work if anything arises on the weekend? They are paid if they do get called in but is there legislation for just being on call or is it more of an employers discretion?