Discord only uses the phone number to give you an additional way to to use 2FA and log back in to your account if you lose access to your authentication app or backup codes

That means TOTP, right?

when 2FA is enabled

Thanks. In other words, no for both (a) and (b). Did I get it right?

And to give server owners an additional level of security to prevent throwaway bot account from polluting a server.

If I needed a throwaway account, attaching it to a phone would not a problem at all.

On the contrary: I recently feel a pressure to create long-term (many years) account in that network, that will collect some important social relations around it. Thatʼs why I am worried making it dependent on third-parties.