If it is “equally valid”, why didn’t it succeed in stopping spam? Is the reason it failed maybe equally valid when applied to phones?

My guess is that - with email, both the sender and the receiver must be using clients that are capable of negotiating the challenge-response wall.

Interestingly, with phone service, the recipient doesn't need anything (other than to tell their phone provider to turn it on if it's provided as an option.)

Of course, this doesn't completely answer your question but it does mean that the decision and implementation of the technology needed to provide this kind of wall is in the hands of phone companies rather than the unruly mass of ignorant individuals who use email.

But then, the phone companies who could implement such a scheme probably don't have a financial incentive to do so.