Surely that's the responsibility of the providers, though? Apple can improve the situation a bit, maybe, but you'd really need to get AT&T & co to crack down on it to have any chance of solving it for good.

I know that I've had approximately zero spam on my German number (that I've had for ~2.5 years) - I'm sure why, whether I'm just lucky, or whether it's much more under control here. My UK number definitely had problems with spam, though. Maybe a couple of spam calls a week.