Besides being trivial to enumerate, every service and their mother is asking for a phone number, from physical stores/rentals/hotels who sometimes reject you if you say no, to the countless online services either asking for it or requiring it. I must have given away my phone number to hundreds if not thousands of entities after having it for more than 20 years.
> None of this is particularly difficult technically. Even simply slapping x509 certs on calls and having some basic filtering would achieve a lot.
Slapping x509 certs on probably some of the oldest telecommunications infrastructure in the world (both in terms of devices using it, and devices enabling it) wouldn't be "technically difficult"?
But I've never worked in telecommunications, maybe I'm overestimating how large piece of work this would be.