At one time, if I got one of these calls, I just used to leave the phone off the hook for 20 - 30 minutes. I understood that his meant that the call centre couldn't call anyone else - but I'm not sure if this works any more - anyone know?
On one occasion, after being driven mad by these calls, I pretended to be my to be my Transylvanian assistant, Igor (who had very poor English). I then pretended to struggle with Igor and got back on the phone as myself (with me so far?). After a few minutes I mimed being seriously assaulted by Igor and flung the cordless handset around the room. The person on the other end merely said, " and now Mr B. - about these replacement windows ...". So, obviously pretending to be a homicidal Transylvanian doesn't work ...
I also signed up for the UK Telephone Preference Service (TPS) and this has cut down the number of marketing calls I receive considerably. Nevertheless, when I complained about a couple of calls, that slipped through the net, the TPS claimed that they had not received my registration. Luckily I had scanned in the initial letter that they sent me and just e-mailed it to them so they couldn't get out of their obligations that easily! It would appear that, these days, everyone is either a crook/conman or incompetent!