Prospective customers will be able to test out the iPhone 6 on T-Mobile's network sometime in the future. T-Mobile

T-Mobile US Inc. (NYSE: TMUS) is among the many mobile networks that will be offering the iPhone 6 come Sept. 19. The carrier also says prospective T-Mobile subscribers will be able to test out the iPhone 6 for free, sometime in the future.

T-Mobile’s Test Drive program, which it announced in June, allows customers on other carriers to “cheat” on their current mobile network by trying a T-Mobile powered iPhone 5s handset for one week, free of charge. At some point, the network plans to switch from offering the iPhone 5s to offering the iPhone 6.

“We don’t have a definitive timeline, but we do intend to switch over in future,” a T-Mobile spokesperson told Recode. “There are a variety of factors that affect that timeline, [including] product availability.”

The iPhone 6 by Apple Inc. (NASDAQ: AAPL), which is currently available for pre-order, will initially release Sept. 19 alongside the iPhone 6 Plus phablet. Additional countries will receive the devices starting Sept. 26. Demand for the iPhone 6 is at an all-time high with more than 4 million units sold through pre-orders as of Monday. Analysts expect the iPhone 6 to surpass 10 million units sold during its first weekend of availability, which could equate to scarce supplies available for programs like Test Drive.

But T-Mobile has other plans for the iPhone 6 in the meantime. The network introduced its voice-calling-over Wi-Fi service and showcased its VoLTE network at its Un-carrier 7.0 press event last Wednesday, one day after the iPhone launch, where Apple announced the iPhone 6 and the iPhone 6 Plus would support VoLTE and Wi-Fi calling. T-Mobile CEO John Legere has noted the carrier’s effort to raise awareness of the iPhone on its network, since T-Mobile is still not well known as an iPhone seller.

T-Mobile plans to upgrade Test Drive iPhone 5s models to iOS 8 as early as next week. Apple's latest operating system will be available to the public starting Sept. 17.