I had a cold caller who insisted he HAD to have my postcode before he could remove me from his database - I wasn't prepared to hand over any more details to him, but he carried on arguing his point until I put the phone down on the table next to me - and he carried on talking to fresh air for an impressive length of time before hanging up.
5 minutes later the phone rang again, and I just had a feeling that it was him again, so I answered the phone in french, and carried on talking in french, insisting that I spoke no English and couldn't understand a word he was saying, until he gave up and hung up on me.
I am usually much nicer to cold callers - I do see that they are just trying to earn a living, and if they are polite to me, I will be polite to them, and often have a bit of a laugh with them - as I did with the dyson service package salesman - but this chap was arrogant and unpleasant, so I felt no compunction about wasting his time.
Interestingly I have never had another call from that firm, so clearly he didn't need my postcode to remove me from their system!
I think treaclesoda's point about the effects, and the potential risks, of these calls to the elderly and vulnerable is the most important that has been made on this thread. I think that the telecoms regulator needs to do much, much more to prevent cold callers from calling elderly or vulnerable customers. It shouldn't be up to the customer to procure a phone that will block callers who don't record their number - there should be some system whereby your phone provider can prevent any cold calls from going through to such customers.
Like another poster, I can't get rid of my landline, as I have dire mobile reception here - we've tried Virgin, 3, Vodafone and T-mobile, and none of them are any good, down our road. If I want to send a text, I have to go and stand on the patio wall in the back garden, or on the front doorstep, and wave my phone in the air. Texts can get into the house though - which baffles me. How is it that I can receive a text, and yet when I try to reply from the exact same place, I can't get a text out of the house??