I was just in the AT&T store the other day doing a BOGO. The rep said I could pay off the one phone I was actually paying for once the credits for the free phone started kicking in (takes three cycles I think it was). He said once I did that, the "free" phone would keep getting a Next credit for the life of the Next contract. They want the assurance the "free" phone stays active for the 18 or 24 months, so it continues even if the other phone is paid off.