If such companies provide contact phone numbers, you might try to look the number up on http://www.tellows.com or http://tellows.co.uk - depending on the country you live. It should work woldwide, though. It´s an anti-scam and anti-spam website where everyone can leave comments on suspicious numbers. Basically it's free reverse number search. Turned out to be the best spam prevention everytime I was receiving these cold calls.