USCC uses either band 12 or band 5 for LTE depending on where you are, Verizon and AT&T are using band 13 and 17 respectively (although they, AT&T, have promised to start using band 12). I would assume that nTelos would use band 25 but I could be wrong about that.
Time tables are difficult to predict but either way, you would need a device that supports both band 12 and 5 to use USCC's LTE.
See above but also add in C Spire who is primarily in Mississippi, I believe.