This seems like an incredible deal. I'm still trying to find a catch, but so far none (I don't mind a 2-year contract). First I thought that the offer would only apply to customers on the more expensive T-Mobile ONE plan, but when I called them, they confirmed that the offer applies to "Simple Choice" customers as well.
No, joke. It's a solid deal. Just make sure you identify exactly which phone you are trading in (e.g. iPhone MLPX2LL/A from ATT / VZW / Unlocked / etc), its GB capacity, the color, and condition.
If I can get an iPhone 7 for $100, that givs me a value of $550 over 24 months, i.e. $23 per month. And that's exactly what I'm paying right now per line on my T-Mobile "Simple Choice" plan (8 lines). So it certainly sounds too good to be true. If someone has found another catch, please let me know.
Let's take my example. On the original iPhone 7 launch promo I traded in an iPhone 6 towards an iPhone 7 128gb. Now on the original launch promo both iPhone 6 and 6s were accepted as a straight up trade. So my 2-year old iPhone 6 was effectively worth $650. Which is a terrific value for it. However, here is the exact breakdown.
The $650 (in my case -- $550 in your case) is broken into two parts. The estimated trade-in value the phone (mine was $165) and the 24 monthly bill credits. After the trade-in phone is confirmed that amount is deducted from the EIP balance. So $650 - $165 = $485. The monthly bill credits are then determined based on that remaining balance. So $485 / 24 = $20.21 credit/mo. 24 months * $20.21 = $485.04 +$165 = $650.04
Now I'm charged $27.09 / mo, but only given a $20.21 credit to cover that. I wouldn't blame you for thinking that's crooked or a scam. But, $650 / 24 months = $27.09/mo -- which is what you'd owe every month if you didn't trade in a phone or traded in broken / busted / no value phone.
But, it's not crooked or a scam. Instead the $27.09/mo is deducted from the remaining EIP balance. Meaning my EIP will be payed off and competed in 18 months instead of 24 (with a final monthly payment of $24.47, instead). In otherworld, the trade-in value of my iPhone 6 reduces the number of months left on the EIP instead of reducing the payment required each month. So I will end up with 6 months at the end where there will still be a $20.21 monthly bill credit even though the EIP is completely paid off. It's weird, but it's t-mobile covering themselves from being scammed by scammers.
Also, no need to worry if you break, scratch, or lose the iPhone 7. The phone is yours. The bill credits will continue no matter what you do with the phone. You can even sell the phone (to a private party) if you want. Just be aware the phone is carrier locked until the EIP is payed off. And you can't sign a second EIP (for the same phone number) until the first (on that number) is complete.