For what it’s worth a Verizon representative told me I can’t unlock my iPhone until the phone is fully paid off paying it off early will forfeit the remainder the monthly trade in credit. I am skeptical if it’s true
That is correct, very clear in their terms, though not sure of the first half.
On the other hand, T-Mobile lets you pay it off at the beginning, and continues the monthly credits.
That’s the industry standard nowadays.
It’s the “new type of contract” to get you to stay with them for 2-3 years (instead of the olden days where you got a “free phone” but signed a 2-3 year service agreement).
The standard is separating the price of phone from plan, and that you'll get credits monthly as long as your a customer.
I'm not sure what AT&T does, but I've only heard of Verizon stopping the monthly credits as soon as the phone is paid off, if paid off early.