I'm not certain, but I think on my phone incoming calls that fail SHAKEN/STIR show the caller id in red rather than black text. I'm on T-Mobile. It also shows "Number Verified" or something like that.
I always imagined that there are certain shady providers ("grey-market Twilio" sort of idea) that just let you run single outbound call/text requests through a giant pool of numbers shared with other customers of the service. Perhaps specifically a bank of residential numbers plugged into banks of regular cell phones, like a residential IP proxy service provider.
It's very unlikely anybody is placing spam/scam calls with regular cell phones when VoIP numbers are easy and cheap to get, and when VoIP systems are far easier to manage.