Since the iPhone 5, Verizon iPhones have been sold unlocked, allowing you to move them to another carrier with no intervention. Older iPhones may be locked and may require you to contact Verizon if you want to use them on another carrier. Unlocking the phone is painless, just call Customer Service (800-711-8300) and ask for an SIM unlock. Verizon’s unlocking policy details are available here.