[degenerate]

What you are missing is, the people abusing the phone number are typically not the owners. For example you can get a phone number from Twilio for $1/mo, and spam people from it. Just like you can upload a copyrighted song to YouTube, or download an MP3 from your ISP-provided internet. Owning the "platform" puts some responsibility on you, but it's not expected that you can stop ALL bad activity in its tracks.

What can be done though, is monitoring for massive calling patterns at the PSTN level, but big telcos are not interested/incentivized in stopping Robocalls because it generates a lot of $$ when the calls travel over the legacy phone network.

[Spooky23]

You punish the accountable parties appropriately.

When Twilio gets a $100k fine for abuse of the $1/mo number, they'll govern the behavior of their customer better and probably eliminate 80% of the bad actors in hours. You could also modify the regulation of interstate carriers to make it expensive to spam entire exchanges with junk calls, or even require licensing to utilize the PSTN. (Which allows you to punish licensees for bad behavior.)

These are all solvable problems, big companies respond quickly to sticks and over time to carrots.