T-Mobile has reportedly blacked out employee vacations between September 20th and 22nd, according to TMO News. Employee vacation blackouts normally occur around big product launches to ensure there are enough employees around to assist customers. The only known upcoming announcement is the Samsung Galaxy Note III on September 4th, but it is more likely T-Mobile is preparing for the launch of the rumored iPhone 5S and iPhone 5C.
Apple is expected to announce both the iPhone 5S and low cost iPhone 5C on September 10th. Apple typically announces new devices on a Tuesday or Wednesday before releasing them on a following Friday. This means a September 20th release date would fall in place with previous Apple product launches.