The caller should already be paying for the traffic, IMO. And since it is more expensive in the US, that should be payed for one would hope. (At least here the caller's operator share some of the money with the recivers') Imagine not being able to be reached once you run out of money, if you pay as you in some way... I have no problem with not being able to call if you'd run out of money, that affects your posibilities to communicate.. But blocking others from calling you, and they wouldn't know when that is the case. nuh..
Okay, my wording is not perfect, but do you see where i'm getting at?
(I get billed for my calls, so it wouldn't affect me that much, but still..)