If your phone number isn't already associated with your primary account, it's possible that you'll unintentionally use it to create a Discord account. Logging into the accidental account is the easiest way to do this if you want to remove your phone number from it:
1. Open a private browser tab.
2. On the login screen, provide your phone number rather than your email address.
3. Just click Forgot Password.
4. To change your password, follow the instructions on the following page and enter the information from the SMS.
5. Register with your account.
6. To access My Account > User Settings, click on the cog icon.
7. Add and then confirm a separate email address from any Discord accounts.
8. Click Remove next to your Phone Number on the My Account page by returning there.
9. If you want to delete this account at this time, you can also click Delete Account.