US Cellular has a native network in parts of the country. They roam onto Verizon in large swathes of the country where they don't have a native network (native network is mostly midwest and parts of the east I think).

THere are also a ton of little regional celluar companies who do have some subscribers but gain most of their revenue with roaming agreements that help the big 3 fill out their coverage. C-Spire and Bluegrass Cellular come to mind, though I think Verizon bought the latter and they are no more.