Find My iPhone must be turned off at the time the device is received. Or the customer won't be able to receive any trade-in credit.

To turn off Find My iPhone, please follow the steps:

Open Settings.
Tap [your name] > Find My.
Tap Find My iPhone, then tap to turn it off.
Enter your Apple ID password.
Tap Turn Off.
For more information, view C2 document 

Did the above information get communicated with the customer?

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Customer advisement

Remind customers of their options to get their device back to us within 30 days:
Bring trade-in device into T-Mobile Retail store and receive immediate final quote or credit.
Mail in trade-in device using the trade-in shipping label (you can find it on their MyT-Mobile.com account) within 30 days of receiving new device.