In the old days they used to print giant books of these "phone numbers" and drop them on your neighbours porches. Odd how things that were common are now security risks.
Makes me wonder how sparse phone numbers would have to be to make spam impractical. Would people use long virtual numbers? Imagine if your friends had your 64 digit phone number, and you would know it was a non spam inbound caller.
Or even better, TOFU, like a Signal call. Or just a Signal data channel over LTE.
un- is a prefix for adjectives, not verbs.
e.g.
Bob decrypted the message and set it to Alice as unencrypted plain-text.
Unlisted numbers are less prone to robo-calling, but most phone companies charge a fee for delisting a number.