T-Mobile put out a press release saying that the company has never seen such a high amount of sales in a single day. On Friday, when iPhone pre-orders opened, the US carrier says that the volume of sales (pre-orders) shattered previous records. In the first four days, the iPhone 7 and 7 Plus have sold about 4 times the amount of iPhone 6 and 6 Pluss in the same amount of time.
As well, T-Mobiles continued efforts to shake up the wireless industry in the US have also brought more subscribers who ported in (switched from another carrier) than it did in Q1.
T-Mobile has recently gotten rid of tiered plans for good and only offers a single, unlimited plan (with some caveats, of course).
T-Mobile is also offering a free iPhone deal for anyone who trades in a working iPhone 6 or above. Up to $650 will be paid to you while you finance the phone in the form of monthly statement credits. The free-iPhone deal must have also contributed to the vast number of iPhones sold at the carrier. Likewise contributing to the amount of people flocking to switch to the carrier.
As well, the carrier says that the most popular iPhone 7 pre-ordered is the matte black finish. (Which happens to be my favorite as well). All of the above, and the phones are not even out yet.