Only GSM phones have a SIM card and can be switched between carriers if they are unlocked and have a clean EIMI number. Do some research online.
iPhones 6 come unlocked no matter what carrier you purchase the iphone from. It's the Sim card that is activated, not the phone itself on the newer iphones... some new government regulation.
I purchased an iphone 6 from Verizon with a 2-year contract 2 months ago, immediately cancelled the service with Verizon, paid the ETF, & switched out the sim card at an Apple Store of my new Verizon iPhone 6 to my current At&T account, Phone works fine, there is no difference. The Apple store gave me the nano sim card for the iphone, All iphones have sim cards, the 6 uses a nano sim so mine sim from my old iphone was too large, or else I would have been able to switch them out on my own, or I could have purchased a nano sim from AT&T, but Apple Store gives them out for free. All iphone 6 come unlock & are transferablle from one phone service to another, great if you travel abroad & have multiple phone accounts. Will work with any iphone 6, which all have sims, unless your with Sprint. Old rules of phone activation no longer applies since it goes through the iphone sim now.
I did a ton of research on Apple forums before I purchased the $600 iphone 6 from Verizon that I was planning on switching immediately to At&t, too much money to play around. Saved over $200.