Ok, not a question but may be of help to someone out there. I've been plagued for about a year by my phone ringing, and listing 'Unknown Number', and when I answer I get a ding-dong sound and it hangs up. Phone company not worth a damn in tracking this.
Anyway, after a while I realised that after a few weeks of these calls, each time Sky would ring with an offer. Couple of weeks back, calls start again, then this evening, Sky call with an offer! I confirmed with the operator there that they do indeed have an auto dialler that keeps ringing, not until you answer, but until one of their people are ready to speak to you. I've removed my number from their system.
Also confirmed with some workmates that they've been getting these too, so hopefully this will help out someone out there.
Not a solution as such but if you keep getting cold calls from Sky it could be because when you filled in the registration form and ticked a box saying you'd like to recieve offers from them. Anyway my advice would be to contact Sky and say if it happens again you will terminate you account.