Additional features, settings, or issues not covered by specific Microsoft Teams categories
MFA = Multi-Factor Authentication.
It's what you use the Microsoft Authenticator app for. Rather than choose to receive the notification through the app, change it so it sends you a text message.
You can change your MFA preferences at aka.ms/mfasetup, and sign-in with the account you are using with Teams. You may need to open that link in a new private browser window.