I had a Look at the Maps and your just north and east of Columbia which is covered pretty good)  but your correct you just outside on the fringe or the Extended network which really only goes just a few miles outside of Columbia) My dad is a Truck Driver and when he went through that area he could not call us what so ever the coverage is very thin around that area.. I think either US Cellular or at&t has a strong hold on that area, with I -70 going through that area coverage should be better..

I would as Tidbits advised take the phones back.. Sorry they didn't work out and it's sad that the Reps led you to believe the coverage was there.. Good Luck b33