"This really depends. If you have responsibility for a production system that is supposed to be running 24/7, of course Ops will and should call you if there's a problem.
But if you have no such responsibility, you can route to voice mail."
That is an excellent point.
I remember long ago when I was the most junior process engineer at a film plant I once got a late night call.
Production guy: How much does a cubic foot of water weigh?
Me: (Half asleep) About 1000 kilograms per cubic meter
Him: How much per cubic foot?
Me: 64 pounds
Him: Uh Oh.... click
I never did figure out who he was or what he had done... they apparently covered it up nicely.
My calls are much less alarming. Usually it is a Tivoli alert that some file system has reached 95% capacity. I tell them which log files to delete, and go back to bed.
But I really hate it when some guy in Singapore or Tokyo is getting a strange error message.