The mammoth three-year-old government antitrust case against the American Telephone and Telegraph Co. was put on hold briefly last month - because the phone company forgot to pay its phone bill.

It seems that AT & T set up an office in the Justice Department building at 521 12th St. (known as the Safeway Building) to look over government documents in the case, but failed to pay the May office telephone bill to the Chesapeake and Potomac Telephone Company.

C & P Telephone sent out a warning bill calling on its parent company to pay up the $448.08 it owed or lose its service.

"We value you as a customer and want to continue to serve you," read the computerized bill form C & P to its parent company. "However, unless payment of $448.08 is received by Jun. 20, we will have to interrupt your service."

The bill was addressed to "A T & T," at 521 12th St., NW.

The bill must have been "taken care of," eventually, because a call to the number on the bill reached George Fanning, of the AT & T team at the office.

But Fanning was reluctant to talk about Ma Bell's problem paying its bill.

"I don't know anything about it; I'm very new here," he said. Besides, he added, "I don't believe I want to continue this conversation with you."

A C & P Telephone business supervisor, who would only identify herself as "Mrs. B. Porch," said she could not reveal whether or not the bill had been paid. "I can't give out that information, unless you are the customer," she said.

"Well, it looks like they pay about as regulary as we do," said Justice Department antitrust attorney Sinclair Gearing. "The only difference is the C & P treats the government better - I don't think we get late notices. And they never cut us off."

Ironically, one of the actions sought by the government in the AT & T case is to force the company to divest itself of its local operating subsidiaries, like C & P Telephone.

"But with subsidiaries like that, who needs a government antitrust case," said one Justice Department attorney.