But you have to do something in the next 30 minutes (responding to the email) in order to not have to do something in the next 30 minutes. There's no "I'm too busy for this" default (except not taking their call, I guess. That's probably what I would end up doing.)
Same here. And I would treat it like any other cold call: if I have time I'll waste some of theirs, and if I don't I'll get them off the line quickly with no care for how rude they might think I am.
What if you're too busy to bother reading the email in the first place? What if you signed up, and then instead of fishing out the confirm link, you got distracted and went off to do something else? Now someone's calling you and you don't know why.
It certainly asks for time in the next 30 minutes, but you're free to do it async on a longer timeline.