AT&T’s idea of “unlimited service” has its limits.  Five gigabytes to be exact, as some customers are now learning.

Weeks after promising AT&T customers enrolled in unlimited smartphone data plans that they could keep them, AT&T is now calling some subscribers of an earlier unlimited plan, telling them they need to limit their use of the “unlimited service.”

AT&T’s Data Connect Unlimited plan was discontinued by AT&T back in 2008, but the company promised current customers they could keep their unlimited plan.  But now, the company has started calling customers when they exceed 5GB of usage during a month.

The Washington Post reports AT&T has been sending mixed messages to customers, and is cracking down on those customers exceeding the company’s arbitrary limits.

“We’ve had a small group of customers on a DataConnect 5GB plan who were not being charged for overage when they went beyond that limit,” she wrote. “We’re now working to bring their accounts in line with the policy for the other DataConnect 5GB plan subscribers.”

Clark added that users who had signed up for AT&T’s earlier Data Connect Unlimited plans (which it stopped selling in 2008) could keep using them, but if they made “certain changes to their account” — for instance, transferring it to a new line — they would have to sign up for a new $60 plan with a 5-gigabyte usage cap.

That comes as news to several AT&T customers who have been in touch with the Post, who were switched, without permission, to limited service plans when they made minor changes to their account or were told AT&T was going to end unlimited service for all AT&T customers.

Rob Pegoraro, who writes the Fast Forward column for the Post, notes AT&T’s customer-care staff seems a little confused about these matters. He advises users with old, unlimited-data plans should be prepared for lengthy calls to customer service — and keep careful records of their interactions with the company.