Many organizations who run like this have enough staff to rotate in and cover a 24x7x365 availability.

Off-hours / unsocial hours are there for triage until the full staff can come online and address the issues.

Also, the week of your rotation where you "wear the pager" means that you will often have the "on-call" hangover day. Essentially that is a day off of rest as to not exceed the 40/hrs week.

Obviously this differs company to company and country to country.