In the cloud enabled world where everything runs 24/7 someone behind the scenes is usually on the hook to make sure that when things break off hours that a human reacts. On call is often a team based rotation of a few days to a week with varying degrees of guarantees. Some teams have SLA’s that require a reaction immediately, others can tolerate more graceful degradation. In any case at some point someone gets the dreaded page from PagerDuty.