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 phone can come SIM-locked or SIM-unlocked, depending where you bought it.
You can use SIM-unlocked phones with any mobile service provider. All phones in the Google Store are SIM-unlocked.
Some phones are SIM-locked for up to 2 years by the mobile service provider that sold them. In this case, a 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 phone before the end of your sales contract, you can contact your mobile service provider to discuss your options.