Always. Every place that I have worked in my career has been a 24/7/365 shop.  Smaller to midsize companies where I was the Help Desk Supervisor, and the Junior DBA, as well as the second network admin and primary phone system support person.  So even if I had a weekend or day off from the on-call rotation, I would still get called.  Thankfully then it was most often my own team calling so we could resolve the issue quickly as they had already done the triage for the situation.