In Teams for your personal life, when trying to sign in or create a new account, you might receive a message "This phone number is already taken. Sign in with that number or use a different one". This means that your phone number is already associated with another Microsoft account. There are a few things that can help with this message:
You can use your phone number to sign in since it’s already a Microsoft account. Select Cancel from the message and you will be taken back to the sign in screen. Enter your phone number and follow the prompts to sign in.
You can move your phone number from the Microsoft account it is currently associated with and add it to a different one. Learn more about how to change the email address or phone number for your Microsoft account.
If you are having issues with your sign in or password, please visit Forgot Teams sign in or password.