I've been thinking through possible next steps for telephony as PSTN dies its slow death under spam.

Controlling a VOIP system, in which callers are challenged in different ways, is one approach. One potential challenge would be to issue random, non-sequential, and sparsely populated "extension" numbers, of say 10--20 digits (10 billion -- 100 quintillion values).

You'd offer a distinct value to each individual contact. Those contacts would be permitted to contact only that "extension". This creates a 1:1 pairing of caller to permitted contact number.

Coordinating generation and acceptance is probably the most challenging piece of this. Callers who might attempt contacts from multiple number is another.

(Which itself suggests assigning an extention to a set or range of numbers, e.g., a healthcare network, school or university system, or large business / organisation with which you engage frequently.)

Friends-and-family would have an easier challenge (whitelisted numbers, some in- or out-of-band validation, or simply being requested to enter the last four digits of their own number (robocall spoofs would probably fail that for at least some time.

Effectively, your single public "published" number serves as a gateway to your own telephony network, into which you can establish both valid numbers, restrictions on same, and challenges, as you see fit.

For humans you want to be able to call you, and you are in control of the PBX receiving the call, and the contact card you are giving them, you can give each person a custom extension, and then preprogram that extension into the contact card/phone number you give them, using a comma to add a delay.

Yes, exactly, and I'm aware of that. Your comment should be helpful to others and is appreciated on that count.