FYI Request the unlock online. I have two iPhone 6's both on NEXT and both are not paid off yet.
I requested an unlock via
their website. with my iPhone 6 and it was unlocked immediately.
I then requested for my wife (who is going to Japan in March and wants to use her phone) and it was denied because it said it was in contract.
I sent in a second request and it said it was denied because of NEXT. I sent in a 3rd request and they said, congratulations your phone is now unlocked.
I tested both phones with my sim out of my work phone (verizon).
Bottom line, request on their website and then request again and again and you will get it unlocked for free.