The Times reports that caller ID, widely believed to be the most effective method of detecting annoying telemarketing calls, often provides misleading information about tech-savvy telemarketers – the practice has been dubbed "call laundering." Sounds dreadful.
I hate landline phones – and, in large part, I dislike telephone calls, particularly when I'm unaware of the caller's identity. While I do my very best not to be too rude to telemarketers, it's best to end things quickly: a quick "this isn't applicable" generally does the job, prior placing the phone in its cradle before anything more can be said.
(Just as a side note, I've had posts about this before.)