C Spire is a MVNO Operator
FYI - C Spire (Previous Cellular South, Inc) is a MVNO operator, not a true carrier. (They do own some AWS Spectrum).
MVNO stands for Mobile Virtual Network Operator. Basically, they're companies like TracFone which offer cell phone service on the big carrier networks under their own brand. The MVNO cuts a deal wholesale with Verizon/Sprint/AT&T/T-Mobile for traffic on their nets, then turns around and sells that capacity at retail to customers.
Usually MVNOs try to target people who aren't being served by big carriers, or who the MVNOs think aren't being served. For instance, there's Jitterbug, which is cheap, low-usage phones with large-print buttons and displays for the elderly (Is that still around?). Or there's TracFone and it's sub-unit Straight Talk, which do cheap pre-paid. Most MVNOs are, in fact, brands for pre-paid service.
Some other MVNOs have tried to target other demographics, even "high end" users. Like Amp'd Mobile, which was one of the first providers to offer an unlimited talk plan. (Now deceased.)
The upside of an MVNO is that they'll offer deals you don't get from the big carriers. The downside is twofold. One, you usually don't get any smartphone options. Two, they're not as reliable as the big boys. Take Amp'd for instance--they got started in 2005, and were out of business in 2007. So even if you find something which works for you, you might find out it doesn't last. The only way MVNOs can make their plans more attractive than the other carriers is by lowering their margins, which doesn't lead to great revenue numbers.
Today, the majority of US MVNOs run on the Sprint network, although T-Mobile, Verizon, and AT&T each have some. (And C-Spire is on Verizon.)