Again, AT&T. Now, this isn't putting down AT&T, in general, it just relates to me. Here USCellular and Verizon are the only networks that are reliable.
Even with an unlocked phone and a SIM card, one is stuck with very little alternatives here. The iPhone, however, is tied to AT&T. I don't believe it can be unlocked. The cell phone business is a complete rip off. These phones should all be unlocked and one should be able to use any network. I have a Sharp TV. I can use cable, Dish or DirecTV. It is not like I have to use Roadrunner, because I have a certain TV.
Also, if I could use iPhone, I wish they have iPhone Business. They probably never will, because Apple tends to cater to the crowd that likes all of the entertainment stuff. I cringe at having youtube on my phone. You can't even find a smartphone without a camera. I would buy the first fairly good phone that advertised, "Hey, no camera." Even my HTC has music, camera, this and that. That was where Blackberry had its market, but now even they have moved in a more entertainment direction.