Unlocking the iPhone is attractive to the globetrotting business owner or employee, as she can make use of local cellular networks without paying expensive international roaming fees to her home carrier. Be warned, though: you could be putting your personal and business data at risk because a used unlocked iPhone was almost certainly also jailbroken. While Apple does now sell unlocked new phones (at a very steep price point) and you can unlock an existing phone if your carrier supports it, be wary of any unlocked iPhone for sale.
Warranty
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Most unlocked iPhones do not have a warranty because they have been jailbroken. Apple considers jailbreaking a violation of its End User License Agreement and will void your warranty: "Apple may deny service for an iPhone, iPad, or iPod Touch that has installed any unauthorized software," the company said in a February 2013 support document on the subject. Without a warranty, repairs and/or replacements could be costly.
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Security
Unlocking the iPhone through unofficial channels requires workarounds that disrupt the security features of the device. Modified iPhones may run the risk of disclosing personal information, according to Apple. Malware and viruses may also be present on the device, which further enhances the security risk. You never know what the jailbreak and unlock process might be changing or installing on your device, and it could be installing malicious code, security firms like Trend Micro warn.
Instability
Unlocked, jailbroken iPhones may not operate correctly. The phone may be unstable, resulting in data corruption and loss. Apps may also run slowly or crash often: Apple says native apps like Visual Voicemail, Weather, and Stocks may stop working. The unlock process has been known to cause issues with voice and data connectivity, and/or cause the internal GPS functionality of the device to become inoperative. Since the iPhone's software is operating outside of normal parameters, battery life may be significantly shorter than advertised.
Considerations
Apple sells officially unlocked iPhones at the Apple Store. These phones come with a warranty but are expensive: as of the time of publication, the cheapest model was $450.00. That was also the oldest iPhone model still on sale. Current model unlocked iPhones are far more expensive at $649.00. If you wish to purchase an unlocked iPhone, purchase a new model from Apple directly rather than purchasing a used model that has been unlocked by the owner. Alternatively, if you already own an iPhone and just want to use it with a different carrier, see if your carrier supports unlocking.