SIM lock, that's why.
I've been in the smartphone game for 4 years, and I always thought CDMA was CDMA was CDMA and no two ways about it. I knew LTE was GSM, but I thought that was only for data. Now I'm hearing from the US Cellular crowd (they're a regional CDMA operator) that their phones can be SIM unlocked and can be used on AT&T and T-Mobile. As in, voice as well as data. Naturally, I called BS, but supposedly, they're doing it. Thing is, you have to be out of your contract. And then go into the carrier and ask them to unlock the SIM. If you're still on contract, they'll tell you to go pound sand. It used to be illegal to do it at all, but President Obama signed a bill that legalized carrier/SIM unlocking. So now they can do it. Whether they will or not... that might be a problem. But US Cellular, for their faults, is a 'friendly' carrier and they tend to look out for customers... sometimes... so I guess some of their customers are having good luck with that. Sounds like BS; I agree. But that's what they're saying.
HTC One M8 / Verizon / CleanROM