Use your Pixel phone on any mobile network
To use a phone on any mobile network, not just the phone seller's network, the phone must be SIM-unlocked. Your Pixel phone can come SIM-locked or SIM-unlocked, depending where you bought it.
SIM-unlocked Pixel phones
You can use SIM-unlocked Pixel phones with any mobile service provider. All Pixel phones in the Google Store are SIM-unlocked.
SIM-locked Pixel phones
Some Pixel phones are SIM-locked for up to 2 years by the mobile service provider that sold them. In this case, a Pixel phone can get mobile service only from the seller's network until the seller removes the SIM-lock or the contract ends.
If you'd like to remove the SIM-lock on your Pixel phone before the end of your sales contract, you can contact your mobile service provider to discuss your options.