Apple works with network providers around the world so that iPhone users have access to a cellular network. In the United States, we struck a groundbreaking deal with AT&T in 2006 that gives Apple the freedom to decide which software to make available for the iPhone. This was an industry first.
There is a provision in Apple’s agreement with AT&T that obligates Apple not to include functionality in any Apple phone that enables a customer to use AT&T’s cellular network service to originate or terminate a VoIP session without obtaining AT&T’s permission. Apple honors this obligation, in addition to respecting AT&T’s customer Terms of Service, which, for example, prohibit an AT&T customer from using AT&T’s cellular service to redirect a TV signal to an iPhone.