How could we design a Captcha equivalent for phone spam?

Many Internet sites use Captcha to make us prove we’re real people and not spiders, spammers, webcrawlers, and address harvesters before we can enter or post.

How might something work that would do the same for our phones? What kind of device could we install or select as a software option to block bots and spam phone calls from reaching us?—something that would make the caller prove he or she is actually alive before the call comes through?

Seems to me there’s a potentially profitable invention or innovation there—something different from whitelists and filters that are based on phone numbers.

Or does it already exist?