I got the 6+ Verizon model. I stuck in my 5s SIM from AT&T and it works. But every now and then I get a text from AT&T saying I need to upgrade my SIM chip to get features I'm missing. I thought I would bring this up again as maybe something just happened as I got another one of these text today. I brought my iPhone 6+ to AT&T. The person there tried to put in a new chip into my phone. He couldn't get it to work. He said it's because the phone is a Verizon phone and won't work. I showed him it's working with my 5s SIM card. I asked what am I missing? He couldn't answer and said it shouldn't work as they don't allow a phone form another company on the network? What? Is that true? Should I be able to get a new chip? What feature am I missing?