Leave it to tmobile to take something that already exists, make it worse and then promote it as earth shattering.
You can return phones for up to 14 days with most carriers so test driving a 5s while they place a $700 hold on your credit card is not awesome, it is chicanery.
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These people so giddy for this will also be paying. They will have a $699 hold placed on their credit card. If you just buy the phone and return it after a week the net effect would be the same.
I don't get why so many people this is some new magical door opened to them. Although to be fair that has been the totality of tmobiles marketing the last few years. Taking something that already exists and claiming it is brand new.
Just like you tried out tmobile and found their coverage inferior, so could anyone else, no need for this loaner program, which is them just selling you the phone and refunding the money if you return it and fine you for stuff like turning on find your iPhone.
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Yeah charging people $700 for $700 phones is taking on a lot of risk.