U.S. Cellular is the largest regional wireless provider in the United States, servicing 23 states, and today Google announced its network is now part of the Project Fi super-MVNO. Google says that U.S. Cellular coverage will be rolling out to "all users" in the coming weeks. USC's network utilizes LTE bands 5 and 12, the latter a band common to T-Mobile devices. Band 5 and 12 are, of course, both supported by the Nexus 6, 5X, and 6P.
This announcement brings Project Fi up to three carrier partners: T-Mobile, Sprint, and now U.S. Cellular. If you're curious whether you live in a U.S. Cellular area, this map gives you a good sense of their first party and roaming LTE coverage.
- Official Android Blog