I used to have an iPhone 5 on sprint. To save money I wanted to try to use the existing phone (unlocked) onto AT&T, went into the store and the rep told me it wouldn't work on their network. T-mobile, as well said the phone wouldn't work on their network. Ultimately, ended up buying new phones from T-Mobile and trading in my old devices
I guess sprint is the only phone company who uses a completely different network or whatever that doesn't allow to use their unlocked phones on different carriers... Just my experience I've had with them in the past.
You can try, but with new legislation passed last year, carriers are supposed to unlock it for you. Also, like karenkcoulter said above, the Sprint model is missing LTE bands so you may only get 3G if you switch to another GSM carrier.
The Sprint iPhone 6 and 6 Plus is the same model as the unlocked one. The SAME EXACT model. It's not "missing" LTE bands. It has the most LTE bands of any OF THE iPhone 6 models. The A1586 is the TRUE "WORLD" Phone.
However Sprint will not unlock you for domestic use... That's the reason it can't be used on AT&T and Verizon not because of LTE bands.
Why would Apple use the Sprint version as the unlocked version of it was "missing" stuff?!?