Usually calling a covering T is restricted for "true clinical emergencies." If you find yourself doubting calling, and then got through your situation without calling, odds are it wasn't really an emergency? You can always call 911, too. Again, if you are debating, then you might be able to do enough good self-care to get through.

When you need someone, really, to talk with, such as during a "melt down" or unexpected crisis, I think one tends to not "care" who is on the other end, you just need help! (That's my nshpo)

I have called a covering T, and even gone in to talk with him. I find that putting off until my own T returns can cause me long term recovery when the other T can help me through. Something that I might call about is when I'm in a pain flare; those tend to not wait for a T's vacation.