I'm trying to understand this. I think the way this can work is if I upgrade a line on AT&T Next, I can pay for the second iPhone in full, then get the $650 in bill credits on that AT&T Next line's iPhone each month. If this can work, then I think I might get the deal to upgrade my dad's phone. My parents' phones/lines are all covered by me on my plan, and I just recently bought a Sim Free iPhone 6s Plus, and gave my dad my old iPhone 6 to replace his 5. If I get this deal, I can just get him the iPhone 6s, get another new iPhone 6s, then sell off my old iPhone 6, and give my mom the 'free' iPhone 6s, take her iPhone 6 and keep it for backup.
*EYES GLAZED OVER*