The SIM card Alert is part of the Anti-theft feature and will let you know the phone number of the person who stole your phone.
How does it work?
If the thief takes off your SIM card and switches to a new one, the Anti-Theft function will recognize that the original SIM card has been changed and immediately send you an e-mail stating that the device’s SIM Card was changed.
From there, you will be able to see the phone number related to the new SIM card inserted on your device so you can share it with your local authorities.
* Note: To enable this feature, you must have a Google account.