I'd like to upgrade to an iPhone X – as soon as one's in stock. Today, I stopped by my local Apple Store and spoke to an employee about which one I should get. For my last few versions, I've had unlocked, contract-free iPhones with AT&T until I switched to Cricket. The rep at the Apple store told me two key things. First, that all iPhones are now unlocked (and have been for a while). Second, I'd have to get one of the iPhone models that work with Verizon or Sprint. That didn't make sense since Verizon and Sprint are CDMA while AT&T, Cricket (which is owned by AT&T), and T-Mobile use GSM.
When I got home I called 1-800-My-Apple. The CSR referred me to Apple's LTE carrier support page (which only makes it clear which carries the phone may work with, not a particular model). While I looked that over, he explained to me that Apple's GSM iPhones only support GSM, whereas their CDMA iPhones support both. That now makes sense – I always wondered why the CDMA iPhones had a SIM tray (part of the reason is that the LTE standard also uses SIM cards).
I learn something new every day.