Facebook (I know) is forcing me to revive a code via a phone number to "prove I'm real". I figured this would be a good usecase for relay phone numbrrs, but thus far I haven't revived any codes after many attempts.
That would be more then I'm willing to spend. I'm basically trying to get around their human verification process. The first thing they demand is a phone number to receive a OTP.
What I did find was a service called smspool.net that let's you order a non-voip number to receive a OTP for around $0.25. You can rent a phone number if you want but I was able to get the OTP and get through the appeal process.
If you want to debug, take the phone number you're getting from relay, and go over to twilio and look up that phone number. See if it's attached to a voip service. Many SMS authentication services simply refuse to send a message to a VoIP number
This is why service providers, like in the directory above, exist to provide non-voip phone number access online.