Just get a virtual DID number from something like Voip.ms or virtualphone. There may be other providers out there that use crypto for payment for added privacy, but if all you want to do is be able to keep your real phone # off the grid, these work.
What i want is a phone that is basically a beeper for text messages. Doesn't even have to send. just receive the stupid OTP. I wonder how hard that'd be to make with like a raspberri pi or something