You'll need your iPhone's IMEI number if you want to unlock it and use it on a different cellular provider.
Find your iPhone's model number in the Settings app. If it starts with an F, it was refurbished by Apple or a carrier.
When buying a new iPhone, whether directly from a retailer, secondhand, or through an online seller, it's smart to double-check its authenticity and condition. Not all iPhones labelled new are ...