If you don't want sales calls, then register with the telephone preference service
http://www.tpsonline.org.uk/tps/ They can stop a lot of junk mail too. If you're registered with the TPS and they still call you, then they're breaking the law and you can point this out to them.
What I've found is that I'm now getting annoying telesales calls from call centres in India, in bad and barely intelligible 'english'. I believe that they're using Indian call centres to get around the law (British law obviously not applying there)