Firebase Auth - Unable to Add Phone Number for Testing

Question

I'm unable to add a phone number for testing my Firebase Phone Authentication workflow. See the images below:

Firebase Phone Auth Panel

After providing a valid phone number/verification code and clicking "Add", I get this error:

I've tried to register my device multiple times over the last two weeks with the same outcome. Hopefully someone has seen this before and if so, is there a solution?

UPDATE:

I've tried the following number formats with the same results -

+1 123-456-7890

+11234567890

Answer 1

You need to delete the User phoneNumber in the authentication tab before you can add (or re-add) a phone number as a test phone number