Robert Wright for The New York Times

When Apple releases its next version of its mobile operating system iOS this fall, iPhone customers will have the option to place FaceTime video calls over the cellular network, whereas before they could do so only on Wi-Fi. On the AT&T network, however, that privilege will be available to customers only on a certain type of data plan, which has raised debate on whether or not the carrier is violating government rules.

AT&T said last week that using FaceTime over its network would be a feature for customers of its shared data plans, not customers who have the older unlimited or tiered data plans. Public Knowledge, a nonprofit group that focuses on Internet law, says that by prohibiting its other customers from using the video-calling feature on the network, AT&T is violating net-neutrality rules by blocking a service that potentially competes with its own.

John Bergmayer, senior staff lawyer at Public Knowledge, said AT&T was violating the F.C.C.’s Open Internet Rules, which say that mobile providers shall not “block applications that compete with the provider’s voice or video telephony services.”

“There is no technical reason why one data plan should be able to access FaceTime and another not,” Mr. Bergmayer said in a statement.

AT&T says it has done nothing wrong, because FaceTime is still available over Wi-Fi. “FaceTime is available to all of our customers today over Wi-Fi, and we’re now expanding its availability even further as an added benefit of our new Mobile Share data plans,” said Mark Siegel, an AT&T spokesman.

Being able to use FaceTime over a Wi-Fi connection on an iPhone, however, is not related to how the service is used on a cellular network. This is sure to stir some more debate when iOS 6 is released.

The F.C.C. said Monday that it had not prepared a comment on the matter.

Phone carriers have been known to push the envelope of the F.C.C.’s regulations. Recently, the F.C.C. reached a settlement with Verizon Wireless after receiving complaints that the carrier had asked Google to block apps that enabled tethering, the ability to share a smartphone’s Internet connection with other devices, from the Android app store. The carrier agreed to pay the F.C.C. $1.25 million, and tethering apps will now be available through Google’s app store.