VOIP or just talki'n

So It has finally happend to me, I have received my first SPAM call. When I answered the call a lady in the other end asked me if I was travelling to the US and if I wanted to book a room at their hotell, I said No, and hung up. No more exciting than that.

But the fact that Spam Over Internet Telephony (SPIT) is here can no longer be ignored. The voip industy have for a long time been saying that SIP spam will explode and we have to start protecting our customers from it. Different methods for doing it has been discussed and so far they seem to focus on assuring the identity of the originator of the call. So my question is will this really work in reality. If we would not accept calls that have no originating number then we would not let calls from payphones, some mobile prepaid, international calls from i.e. syria, irac or other with more temporrary telephony solution, be connected. So customers with friends and relatives using any of the previous would call in to customer services and say that their friends can't call them.

So do the operators have to implement different solutions to stop SPIT depending on if the calls come from an IP-interconnect or from the PSTN? Also would we need to handle calls differently depending on if the call originated from the PSTN but was connected to operators net via an IP-interconnect?