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Telephone bills, which contain information for an AT&T customer. The company says now, customers who use too much data will either have to pay more, or deal with slower speeds. - 

Adriene Hill: Sorry to break the news, but it's the end of the line for truly unlimited data on your AT&T phone -- even for the folks who were grandfathered in.

Marketplace's Nancy Marshall-Genzer reports.

Nancy Marshall-Genzer: AT&T stopped offering unlimited data plans two years ago. But people who already had them, got to keep them. So, they could download movies and play video games on their phones to their heart's content.

Fran Caulfield is a telecom analyst at Insight Research.

Fran Caulfield: There could be people that get all their video watching done through their mobile phone or their tablet. But the network gets taxed when you do that for many hours over the course of the month.

AT&T says those heavy users were overloading its wireless network, so it had to do something. The company says these customers still have all-you-can-eat data plans. It's just all you can eat... slowly.

And the company says only about 5 percent of its customers in a given area are affected. But those 5 percent are really mad. Some say their phones slow down so much they're practically useless. AT&T says they can either upgrade to a more expensive data plan, or switch to a plan where more data costs more.

In Washington, I'm Nancy Marshall-Genzer for Marketplace.