The IMEI also known as the International Mobile Station Equipment Identity is a specific number that is used by service providers to identify each device. These GSM companies make use of the IMEI number to blacklist or locate a misplaced or stolen phone. Before you can use your Pixel 2, you have to make sure you have completed an IMEI number identification process for Verizon, AT&T, Sprint and T-Mobile.
When an IMEI number of a smartphone is on the blacklist, all GSM companies will be notified about it. They will not allow it to connect to their network. It’s basically not possible to change the IMEI number of a device and this makes the Blacklisting method an effective one. This is the reason why it is always advised to pen down your IMEI number in a safe place (not on your Pixel 2) so that if you misplace it or it get stolen, you can report the IMEI number and this will prevent your device being accessible to someone else. If you want to know how to locate the IMEI number of your Pixel 2, read this guide.