If you have your 2FA, also called Two Factor Authentication turned on, you will always be prompted to enter your Auth/Backup code in order to log in to your Discord account.
What happens if I enter my phone number, not my 2FA code?
An error message "Enter Discord AUTH/BACKUP Code - Invalid phone number" will pop up. Discord's copywriting for the "Invalid phone number" error should be ignored.
Hence, make sure you have entered your Backup code provided by Discord. If you can't find or don't remember where your backup code is, you can read in detail how to find your Discord backup code via this link - https://support.discord.com/hc/en-us/articles/115001221072-Lost-Two-Factor-Codes