The three things that have to exist
A way to raise an alarm, a person who will respond, and a trigger if neither happens. A phone counts as the first only if there is reception in the areas being cleaned, which is not true of every basement or stairwell. The second fails most often, because the nominated contact is a business-hours role and the work occurs at ten at night. The third is almost always missing entirely.
The trigger matters because an injured lone worker cannot raise an alarm. If nothing happens when a shift does not end, the arrangement depends entirely on the person being able to call for help, which is precisely what an incident may prevent. A simple end-of-shift confirmation with an escalation if it does not arrive closes that gap for very little effort.
