If you bought a phone or accessory in a T-Mobile store, you have 14 days to return it with its box, accessories, and receipt. Not that there's anything wrong with it or that there's a recall. It's the updated returns policy spotted on the carriers website. They're also raising restocking fees, and you’ll likely pay up to $75, depending on your device.
There's now a $5 increase
As spotted by DroidLife, and now reflected on the carrier's support webpage, T-Mobile will now make you pay extra for returning devices when the real-world experience doesn’t match what you expected after buying it. To them, it's only fair, given that they can’t sell it as brand new anymore once the box is opened.

Before this change, the restocking fees cost $20 if your device was originally under $300. If it was between $300 and $599, you paid $50 and paid $70 if it was $600 or more.
Now, there's an increase of $5 on the cheapest and most expensive tiers, while the middle tier remains the same at $50.

There's no official statement explaining the reason for the increase. But it does a good job in discouraging customers from buying phones just to try them and return them. Plus, there's a bit more revenue from returns, especially at the high end where many users fall under.
Still want to make a return? Here's how it works
When you buy a phone from T-Mobile’s walk-in store, you have 14 days from that purchase date to return it. If you ordered it online or over the phone, the timing starts from when it was shipped to you, and you get about 20 days.
To return it, go back to a T-Mobile store or ship it back the way it was delivered. They'll check the device and ensure it's in good condition, along with the box and any accessories that were in the package. You also need proof that you bought it, which is your receipt.
Once successfully inspected, the company automatically deducts a restocking fee based on the price of the phone from the refund. Damaging or altering the product may reduce the refund further or they'll refuse the return entirely. Also, it's not unlikely.
DCC fees have also gone up
T-Mobile is on a roll, and it's not a tumble customers will be happy to take with them. They already had a $35 Device Connection Charge (DCC), and it's the fee they charge when you activate a new line or upgrade to a new device.
Now, usually there is a workaround where you could avoid that fee if you bought an iPhone directly from Apple, even while using T-Mobile financing or promos. Apple wouldn't add it at checkout, and T-Mobile won't bill it separately.

They've closed the loophole, as reported by The Mobile Report. If you now buy a phone from Apple and connect it to their network, you’ll be charged and T-Mobile will add it to your next bill instead. Customers started seeing the fee appear in carts from March 17, but the carrier officially began charging it on March 27.