One thing about the UK is that you never have to pay for incoming calls (unless you're roaming outside the EU), whereas you often have to pay per-minute for voicemail. Consequently, you really don't want them to leave a voicemail, since if they do, the fuckers will cost you actual money, or you'll have the voicemail icon showing and might miss a real message later. So there's some incentive to answer; it might even be worth calling back if you have inclusive minutes so the marginal cost is zero.
I get a lot of ambulance chasers. I generally ask them to remind me about the "accident that wasn't my fault" - when did it happen, exactly? What happened? What do you think my name is? Really I'd like a way to hurt the shyster company in the pocket without hurting the poor kid trying to make a living at a call centre, but I'm not sure how best to do that.