iPhone 7 network locked

I have moved my iPhone 7 to a new carrier from

AT&T.  The new carrier says the phone

is locked and cannot be used on their network. Can it be unlocked? I see many

internet sites that say they will unlock the phone for anywhere from $20 to

$120. The phone is fully paid for so is AT&T responsible for unlocking it

since I bought it from them? What should I do to allow the phone to be setup on

the new network?