Gifted iphone, Unknown Carrier... Need help with unlock
So i got a brand new iphone xs max as a gift at a wedding i was invited to and gave it to my brother as i already use an 11 pro max. When trying to activate it he found out it was locked to a specific carrier. As it would be rude or seem ungrateful to ask the wedding party about the carrier i wish to know how i can determine the carrier so i can contact them and proceed with unlocking the phone for him. Confused on how to go about this and where to start from. Need help please. Anyone?
iPhone XS Max