Hello, I'm looking to have an unlocked iPhone and while searching through Swappa.com, I have more options if I can get an iPhone to unlock than just looking at the unlocked models. My question is among the different types (Verizon, AT&T, T-Mobile, Sprint), which carrier is best at unlocking their phones for second hand customers? I've heard that Verizon phones are already unlocked for GSM? I've also heard that Sprint phones have some sort of Software lock??? Thanks.