Decisions to call the MD need to be made on the basis of, "Does the MD need to be aware at this time. Does the patient need interventions that are not ordered? Tests that are not ordered?" And that decision needs to be made by the nurse caring for the patient. (With input from others if necessary, but the decision needs to be made by the nurse caring for the patient.)
Never on the basis of, "Will someone be mad at me for calling?"
So often I hear, "It's Dr. X on call, she's nice, go ahead and call." The only time the identity of the doc on call matters is if it's, "This MD wants to know this immediately, this one would rather hear when they make rounds."
If you're afraid of being yelled at for doing your job, GET OVER IT.
And don't let a coworker talk you out of calling if you feel a call should be made. I've worked on units where you're supposed to get the charge nurse's permission. Those units will throw YOU under the bus along with the charge nurse if things go south. You're the nurse. YOU have the letters after your name. Make the decision. Get input if needed. I'll often have new grads tell me they want to call, and after discussing the situation, we work out that it's "just ..." or something that doesn't actually need a call once we think it through together. But if you think you need to call, it's YOUR patient being cared for under YOUR license, which makes it YOUR decision.