T-Mobile has been pushing the FCC to change up the rules a bit on the upcoming spectrum auction that's set to take place next year for the 600MHz spectrum. Although they are the loudest, T-Mobile is not alone. The reason that T-Mobile wants a bigger reserve is quite simple, actually. Verizon and AT&T already have the majority of the low-band spectrum (low-band is super important because it allows carriers to cover more area with less towers, and get better in-building penetration), as well as having a ton of money. So theoretically they could squeeze out smaller carriers like T-Mobile, Sprint, and other regional carriers. This isn't what's best for competition though.
This time, T-Mobile has submitted a few more filings to the FCC, using research done by economists. This is actually quite common, as it gives the FCC proof, instead of bias reporting. T-Mobile wants the FCC to raise the reserve from 30MHz to 40MHz in any given market, this means more for the little guys. Verizon and AT&T have continued to argue against this saying it's not necessary, and it'll decrease the revenue that can be made from the spectrum auction. The duopoly, won't be able to bid on spectrum in markets where they already hold 45MHz or more in the low-band region (which is 1GHz or lower), which is actually quite a few markets.
T-Mobile has really turned itself around in the past couple of years, and have been buying up as much 700MHz spectrum as possible to strengthen their network. And in areas where they have already deployed the 700MHz spectrum, you can really tell the difference. There's much better signal, inside as well as outdoors. Which has been a pretty big issue for T-Mobile. This 600MHz spectrum is very important to T-Mobile because it's the last low-band spectrum auction for the foreseeable future - we're looking at a decade or more from now - and if AT&T or Verizon can squeeze out the little guys, they won't be able to last until the next spectrum auction. So to keep competition alive, T-Mobile has been petitioning the FCC to change these rules slightly. And as we stated above, T-Mobile is not the only carrier here doing so.