The phone number is used to create the check url. This check url is returned from the mobile network operator that phone number and SIM card belong to.

The device makes a GET request to the check url, with a cellular data connection. The mobile network operator is able to verify that the phone number used to create the check URL matches that of the phone number assigned to that SIM Card making the data connection request.

And if SIM Swap is a concern, we also have an API that allows you to first check whether that phone number has recently switched SIM cards before proceeding with the verification.