for UK users you can look up TPS (telephone preference service) and register with them for your mobile and landline for free. It won't fully stop it but should reduce the unwanted calls. Also if you answer and say this number is registered with TPS then they behave like a Vampire seeing a Cross... they just want to disappear.
I never get these type of calls on my mobiles... always on the landline, I'm very careful where I use my mobile number. I know if the landline rings its not important.