I'm in the UK, and received a call from "Thomas" who worked for microsoft, apparently, which had detected a virus emanating from my PC. They knew my name, and clearly had my (ex-directory) phone number. Apart from the incredibly crackly line and Indian accent, they sound /very/ convincing. Their MO. is to start off by offering to work with you to identify the virus. A friend's parents were caught out by this. The 'support engineer', talked the user through navigating to the IE Cookie folder and demonstrated that there was personal information in that folder. He then got them to download a 'cleaner' (i.e. trojan). When he asked for payment of £435, my friend's dad had presence of mind to say "Invoice me", to which the 'engineer' hung up.
I've called TPS, OfCom, BT and written to our local MP (recieving a reply from Ed Vasey, no less), assuring me that there's nothing anyone can do, because these scams originate abroad.
Until someone in some power leans on BT to actually chase up these scam calls, it'll just continue, and these criminals will continue to operate with impunity.
BT's best offer was to 'block all international calls'. I suggest that everyone call BT and ask for this feature, if you can.