Apple is said to be in talks with carriers in both the U.S. and Europe about launching a mobile virtual network operator (MVNO). An MVNO service from Apple would allow customers to purchase cellular access directly from Apple, which would lease space from existing wireless providers. Phones would switch networks automatically in order to get the best service.
From Business Insider:
Here's how an Apple MVNO will work: Instead of paying your carrier every month, you will pay Apple directly for data, calls, and texts. Apple then provides you with everything you used to get from your carrier, but the Apple SIM switches between carriers to get the best service. The telecoms companies auction capacity to Apple so it can run the service.
This would be similar to how Google's virtual network service, Project Fi, already operates. Apple's service is still reportedly in the testing phase, and if it does eventually launch, it apparently won't do so in the near future. The company could take at least five more years to launch it completely, and is said to be taking a long-term view with the potential wireless service.
Source: Business Insider