Until now, opearting in different carriers required iPhone users to 'jailbreak' or hack their phones.
It's unclear whether the feature is intended and whether the phones will work with other carriers.
Last year, the first shipments of the Sprint iPhone 4S were "unlocked" and worked on AT&T, but Sprint later issued software updates that turned off that capability.
In the meantime, the iPhone 5 is already sold out in many U.S. stores, just one day after its official release.
Apple's website said phones bought online would ship in three to four weeks. Verizon's website said they would ship by Oct. 19.
Analysts say the company will likely sell millions of phones in the first few days. Apple is expected to provide sales figures on Monday.