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When ordering the phone through the AUP, does it ask for any information from your carrier or have you sign in with them?
 
So if I use the AUP to get the iPhone 7, and then next September, I get the iPhone 7S, and let's say keep the 7S for over 2 years, will I only have paid for half of an iPhone 7 and the full price for the iPhone 7S?

If that wasn't clear, do I effectively only pay for half of each phone if I upgrade each year?

Correct. You don't own the phone until you have made 24 payments for one phone. You are exercising your upgrade option after you have made 12 payments (50%) of the total cost
 
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