Discord might ask you to use a legitimate phone number to confirm your account. You cannot link your phone number to an already-existing Discord account. Discord also don't accept all types of phone numbers.
The procedure will resemble this in some small ways:
1. The Start Verification button should be pressed.
2. You will need to enter a working mobile phone number at this point. Keep in mind that it must adhere to Discord rules and cannot be connected to an active Discord account.
3. Wait for the SMS to arrive on your phone, then enter the verification code as directed.