Yes, or even require a challenge sent to the current line with a grace period, and you get to choose your own grace period up front. In this way, someone can't

Yes, or even require a challenge sent to the current line with a grace period, and you get to choose your own grace period up front. In this way, someone can't jack your line while they know you're on a flight. So, I lose my phone (maybe it's sitting on the side of the road somewhere) and need a new line. Since I can't reply to it my line will transfer after 8 (?) hours of no response to the challenge.