What conditions must my line meet for this to work?

Just one, really: your line must allow forwarding on no-answer. That's true of nearly all professional landlines and mobiles, and most VoIP systems. Only a few prepaid plans and some locked corporate PBXs block the feature — we detect it by dialing the forwarding code during activation, before any payment. If your line fails that test, we tell you plainly and look at other activation modes instead of selling you something incompatible.