Apple on Friday began offering a factory-unlocked version of its popular iPhone 4S smartphone that will work on any GSM network in the world. The company now sells the unlocked phone in both black and white starting at $649 for the 16GB model. The 32GB iPhone 4S costs $749 unlocked, and the high-end 64GB model costs $849. No carrier contract is required with the purchase of an unlocked iPhone 4S from Apple’s online store, and the vendor is currently quoting shipping estimates of between 1 and 2 weeks. Apple also notes that while the device will work on a variety of GSM networks around the world, it will not work with CDMA carriers such as Verizon Wireless and Sprint.