The instinct to bring in a second pair of eyes is good. The risk is that you do it before you have enough context to brief them — at which point you have two confused people instead of one, and you have woken up someone you might not need.

The clean rule of thumb: page a second responder when you would not be confident describing the incident to them in two sentences. At minute one of this page, you can describe it in two sentences. "Payments-API is at 100% error rate in eu-west-1. I am about to check whether we deployed in the last hour." That is enough orientation to act alone for the next five minutes.

If, five minutes in, you still don't know what's happening — then you page a second responder, with a real brief. "Payments-API at 100% error rate, no recent deploy, no obvious infrastructure event, I'm out of cheap hypotheses, can you look at it with me."

That second page is precious. You burn the goodwill of a sleeping colleague. Make sure you burn it on a real ambiguity, not on first-minute jitters.