Back in April, T-Mobile purchased a large chunk of the 600MHz spectrum for $7.99 billion. The move was unprecedented, but the new network should allow for better service in-doors, and should be less congested than competitors' low-band spectrums, which is typically the shared 700MHz band.
Today, the Un-carrier announced that it will begin its 600MHz rollout this summer, and the first sites will be ready for testing. The company says that this "sets the stage for commercial operations later this year."
Keep in mind that while the 600MHz spectrum is going to be fantastic for T-Mobile customers, it won't work with existing hardware. Just for one example, you can check out the LTE bands that are supported by existing iPhones and see that 600MHz isn't really built into anything just yet. Of course, phones with support for the spectrum will be available later on this year.
As you can see from the image above, T-Mobile is licensed for 600MHz in all 50 states, and it has an average of 31MHz of space, which goes up to 50MHz in some areas. The firm says that not only will the spectrum strengthen its LTE network, but it will also be used toward building the first 5G network.