A phone number IS identity these days.

Yeah, so many places ask for phone number that don't really need it that I assume the phone number is a unique identifier used to combine individual's data across websites.

Most of the time I use a made-up 555 number or if it needs to send an SMS to verify, I'll use a free SMS numbers.

Not if you buy a SIM or eSIM anonymously. This is easy in the U.S. with cash in a store, or online (silent.link).