I was having the same trouble for ages. I rang NTL and had Anonymous Callers blocked. Which works because I dont get half as many as I used to. The nice CS Lady also gave me a number to ring to have a block on all Switch Board callers. 0845 070 0707 if anyone wants to give it a go. I haven't used it as of yet.
Now when people phone, if they dont answer first time, I hang up. If they do answer first time and sound foreign, I tell them I'm not interested in mobile phones and they hang up. LoL.
It really is getting beyond the joke though with all these cold callers. I spoke to one about a week ago and he said I could have a FREE mobile phone. I said OK, if it's free, send the phone. He said I needed to sign up to a tariff. So I asked how can it be free if I have to pay for it. Make me laugh. There is no such thing as free.
I've also tried asking them to take our number off the list and not call again but they do. Wouldn't surprise me if they all worked in the same call centre aswell.
Anyway, thats my story. Glad I'm not alone in this trouble but hope everyone gets sorted (and me) soon.