Iphones buying from US carriers to use internationally?

For Iphone 4's am I able to buy an AT&T or Tmobile Iphone and somehow unlock them to use internationally? Is this by jail breaking? Or do I just have to buy the unlocked ones on ebay to be able to use an iphone internationally?

Yes i bought my iphone 4 from att and wanted to use it in france. I found this website online that unlocks iphones called unlockatt.com and they did it for me! I connected to itunes and it worked and my iphone is now unlocked

  • When do US carriers (AT&T, Verizon, etc) lower the price of older model iPhones? Usually when a new iPhone comes out, the previous years model gets discounted by about $100. Does this price change usually happen when the new iPhone is announced, or is the price drop not effective until a few weeks later when the new model actually becomes available for sale?
  • What is a good app to use to text for free internationally for the IPhone? I'm going to Mexico in a week and I want to be able to communicate (text, calls) with family in the US while i'm there. What is a good app to download for an Iphone that would be completely free for me and the other person?
  • What carriers take CMDA iPhones? I have an iPhone 4 that was connected to the sprint network. I just bought it and I'm not with Sprint anymore, I bought it off Craigslist (if that helps) and I really do not have the money to dish out a contract AND a phone. CMDA meaning no SIM slot.
  • What carriers are best with iphones? I plan on getting an iPhone soon, which carrier is best with iphones version, boost, virgin mobile, t-mobile, sprint, AT&T. BTW, the iPhone I'm getting is NOT under contract.