[derefr]

I believe you're thinking phone numbers from legitimate VoIP services like Twilio, or the "texting app" service-providers that build on top of them.

The GP is talking more about phone numbers from purpose-built (usually Russian) "secondary market for other people's credentials" marketplaces, where people sell the use of their own personal phone numbers (usually through cloud remote-control software they run on an old Android device with the SIM in it.)