T-Mobile's LTE network has come a long way since the carrier started deploying LTE, but there is still room for growth and expansion to reach more customers and bring them the LTE support and speeds they want. To further advance their network and expand it out to cast a wider net over more people, T-Mobile has confirmed that they have begun leasing out their LTE network spectrum in an effort to make their LTE network grow. This could work out to a huge benefit for T-Mobile especially if it works similar to the agreements Verizon has for their own spectrum leasing program, which allows Verizon to lease out the spectrum to other carriers who can then build their own networks, with shared roaming between the companies so customers on either side get access to each network.
If T-Mobile's leasing efforts come along with an agreement to share spectrum between leasing partners, (and it's likely it does) it could drastically improve the LTE network coverage for T-Mobile customers who may not have LTE coverage in certain areas. Customers of T-Mobile's leasing partners could benefit as well as they could potentially gain access to roam on T-Mobile's owned and operated network towers. While T-Mobile has confirmed that they are beginning to lease their LTE spectrum so they can expand out their network reach, they haven't mentioned who their leasing partners are at this time.
In addition, they have also kept quiet about exactly how much spectrum they have leased out to partners and how many carrier partners they have as part of this initiative. There is also no detail regarding which LTE spectrum bands were leased out or where the coverage range of that leased spectrum is, so at the time being there is no way of knowing where T-Mobile's LTE network is going to expand out to via this program. While T-Mobile has been rather withdrawn on details for these leasing efforts, they have stated that they're always looking into new opportunities that would allow them to improve their LTE network. Once the leasing agreements are further along, T-Mobile will likely shed some light on the more intricate details of the program.