Probably tmobile, my buddy owns a building for his business and when
tmobile came into the area they made a deal with him which included free
phones. The only problem is that they have such a poor network it is
really difficult to get any value from the deal. Because of that he
uses VZW and the tmobile phones go largely unused. From other people I
know the better carriers don't offer such deals and don't pay the huge
rents that have become urban legends.

What cellular company is hosted on the farm cell tower?

I'd love to.  Where do I sign up?  Our house is close enough to an
interstate that it actually would fill a dead spot or two, if they put a
tower in our back yard.  There's plenty of room back there, too.  We'd STILL
have a huge back yard.  :)  -Dave