AT&T is blocking longtime customers with unlimited data plans from using a cool new feature available on the latest iPhones: FaceTime video chat over 3G and 4G wireless networks.

One of those customers, an architect living in San Francisco, has filed a complaint with the Federal Communications Commission, he told Business Insider.

He and his wife decided to upgrade to the iPhone 5 mainly for the improved camera and FaceTime. He wanted to be able to FaceTime his wife and kids while he traveled. (He asked that we not use his name.)

"I am one of those AT&T 'unlimited'-data customers who bought a plan when the iPhone first came out," he told us. "I have paid a premium for the unlimited data plan even though I don't really use that much data, thinking that some day it would be useful to have."

Previously, AT&T had only allowed FaceTime video calls over Wi-Fi networks. When Apple announced an upgrade to its operating system this summer, it said FaceTime would now work over high-speed wireless networks.

But AT&T said it would require customers currently on unlimited-data plans to switch to new Mobile Share data plans to use FaceTime, or remain limited to Wi-Fi usage. Mobile Share caps monthly data usage at set levels, and charges fees for overages. Video chats use a lot of data, so FaceTime users may get stuck paying extra.

UPDATED: Some public-interest groups wrote an open letter to AT&T announcing their plans to protest its decision, citing FCC rules about how carriers treat data from apps running on their network. They have yet to file a formal complaint, though. One of those groups, Free Press, told us that FCC rules required them to give AT&T notice in advance before taking such action.

A formal complaint is more like a legal process and once filed, the agency would "have to address the matter," the Free Press's Jenn Ettinger told Business Insider. If AT&T doesn't fix the problem to its customers' satisfaction, the groups still plan to file that formal complaint.

But nothing's holding customers back from filing complaints now. So the customer who spoke to us is urging other customers to take matters into their own hands, like he did.

Filing a consumer complaint, he said, was surprisingly easy to do, and he hopes others in his situation will head over to the FCC website and do the same.