There are really only 4 major phone companies in the US. AT&T, Verizon, Tmobile, and Sprint.
There is another type of mobile reseller called an MVNO.
Here is a list of mobile virtual network operators.
https://en.wikipedia.org/wiki/List_o...work_operators
You might think you are using XYZ cellular... but XZY is just reselling minutes from one of the major carriers. XYZ has no phone system. They may not even have a billing system, or a call center or any of it! IOW, they may just be a marketing company with a brand. Some MVNOs handle some of the backoffice stuff.
Here is more info on MVNO.
https://en.wikipedia.org/wiki/Mobile...twork_operator