I am currently an AT&T customer. I'm looking specifically for a T-Mobile iPhone for two reasons. 1) I am going to be switching to T-Mobile and 2) I want to fully pay for the phone so it is unlocked for international travel so I may use a local sim card from another country. If I buy a T-Mobile phone from an Apple store, my understanding is that it will be unlocked. I believe I have read that if you turn the phone on with the T-Mobile sim card still inside, it will be locked to T-Mobile only. Is this true? I have also heard that Verizon is a truly global phone that is also unlocked. Would it make more sense for me to fully purchase a Verizon phone and replace the Verizon sim with a T-Mobile sim prior to turning the phone on? Again, my goal is to be a T-Mobile customer AND have a fully unlocked phone for international travel. Thank you for any feedback you may have.