Here's how this attack works: https://www.google.com/amp/s/www.zdnet.com/google-amp/article/how-i-survived-a-sim-swap-attack-and-how-my-carrier-failed-me/
Immediate actions
- Make sure your primary email address has a secure recovery method and remove your old phone number from it.
- Gmail lets you set "Backup codes" that you can print off to recover your account.
- Change your email password and put it in a password manager.
- Remove any link to the old phone number from your email.
- If you're already locked out of your email, set up a "clean" email account. At this point, your old one is being accessed by hackers and can be used to reset any banking passwords.
- Store this new email password in a password manager.