I hate prank calls, I don't find them funny and I think that someone always ends up embarrassed at the very least, upset and hurt most of the rest of the time.
However, no matter how bad taste the call was, I don't believe there was any way they could have anticipated these events would happen. No one could have; it is unbelievably tragic but it just isn't something anyone could have predicted.
I expect they thought they'd make the call, 'lark' around doing silly impressions of the Queen and Prince Charles and get hung up on by the receptionist, and that was all they expected out of the call.
I agree, YANBU, I feel sorry for them too. They just couldn't have foreseen that this would happen. So while I know what you mean, witchety, and I agree that it is just not funny to call a busy hospital with a prank call, I don't see how this tragic outcome could have been foreseen as a possible consequence.
If anything it is the media's fault for hyping it up so much and making such a massive story out of it.