The new iPhone 6/6 Plus are dual-mode CDMA/GSM regardless of model (ATT/TMobile/Verizon/Sprint). On the 5c/5s, the GSM versions have CDMA disabled but that seems to have changed. The only issue with Sprint is they probably won't unlock phones released before 2/2015. I'm using an AT&T iPhone 6 Plus on Verizon and I've used a Verizon iPhone 5c on AT&T and T-Mobile.