In Teams for personal and small business use, when trying to create a new account using a phone number, you might receive a message "phone number is already a Microsoft account". This means that your phone number is already associated with another Microsoft account. There are a few things that can help with this message:
If you've recently gotten the phone number (or it's an alias for a different Microsoft account and you want to associate it with this Microsoft account), select Sign up from the message and follow the prompts to remove it from the current Microsoft account and move it to the Microsoft account you'd like to use.
You can use your phone number to sign in since it's already a Microsoft account. Select Sign in from the message and follow the prompts. Learn more about how to find your Microsoft account.
If you are having issues with your sign in or password, please visit Forgot Teams sign in or password.