Originally Posted by der_saeufer

The overwhelming majority of providers that say "no VoIP numbers" don't care enough to look up who currently holds the number. They use a list of central office (NPA-NXX) codes to see whose block it came from and call it good. I suppose from the point of view of "is this a real person" that's good enough, since someone would've had to go through the trouble of signing up for service with the cell carrier and porting it out. This is an extremely common issue for Americans living abroad who don't feel the need to waste $20+ a month on an American cellphone plan.

My American number has been on Google Voice for nearly 10 years and shows that it belongs to bandwidth.com, but it was originally an AT&T cell number and works fine for SMS 2-step verifications. Friends whose numbers were created by Google Voice have a much lower success rate.