When the new iPhone 7 dropped in stores back in September, T-Mobile quickly took advantage of the hype by offering it for free in exchange for another smartphone.

For this year's Black Friday Sale, T-Mobile is bringing back the popular deal. Like before, the 'free' phone promotion comes with an asterisk. The hitch is that customers will again be asked to trade in their old smartphone to get a new iPhone 7. But the carrier will not accept just any phone and a payment will be necessary.

Interested parties may trade in their iPhone 6 for a 7 provided they part with $100. Those with an iPhone 5, 5cC or 5S will need to pay $350 before getting their 'free' phone. Different brands will also be accepted as long as the phone is more expensive than the iPhone 7. This simply means, those who recently got a high end Samsung or LG smartphone, for example, can exchange them for the latest iPhone. Considering iPhones remain one of the most popular smartphones out there, this is actually a good deal.

The deal will only be possible if the old phone to be traded has been fully paid. Another condition that has to be met before the deal is approved is that the customer should make T-Mobile their carrier.

T-Mobile is not the only carrier that has been making a lot of noise lately. According to Fortune, the market for carriers is "no longer expanding like it did a few years ago". T-Mobile and Sprint saw the number of their regular monthly phone customers increase while Verizon and AT&T suffered losses. This forces carriers to think of other ways to get new customers.

One of those ways is through deals and promotions like Sprint's buy one, get one offer on the Samsung Galaxy S7 phones. Verizon, on its part, is offering the new Google Pixel since the two companies agreed on an exclusive deal. The deal slashes $200 off the $650 retail price of the Pixel. AT&T, meanwhile, is offering a free iPhone 7, Samsung S7, LG G5 or LG V20 for those who switch from another carrier.