No, you didn't fully pay for the phone. The carrier did. You'll be paying back the carrier over time or with an ETF.
Paying the ETF for a phone, is no different than canceling a car loan by giving the bank the remainder that you owe in one payment. Both are repaying the loan.
The math is simple:
I sign a contract for an iPhone for 2 years. The carrier pays Apple ~$600 for the phone. I pay the carrier: $200 up front, plus a $30 upgrade / activation fee, plus even if I cancel after only two weeks (I'd have to give the phone back before then!), I'll still have paid at least $20+ in monthly fees.
$200+30+20 = $250 up front.
That means I owe the carrier $600 - $250 = $350 payoff. That's what the ETF covers if I cancel right away. The ETF goes down each month, because I'm paying back the carrier from within the service fees.
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Phone companies would be out of business very quickly if they didn't get back what they paid for each phone. High phone prices are already killing them. For example, AT&T took a stock price hit the first time it was revealed that they had more than a half billion dollars tied up in iPhone subsidies (customer loans). Apple's prices are a major reason why carriers are pushing non-Apple phones, or in some countries have even stopped subsidizing iPhones.
With Sprint, you can't just simply buy the phone at full retail price and not be in a contract. They require you to sign a 2 year contract when buying the phone when first starting service. I'm sure other carriers do this as well now. Maybe outside of America though. Even if you pay full retail price of the phone, you are still getting charged the $30+ data fee regardless on if you sign a contract or not.
I'm not buying what you're selling here..