American Telephone A. Telegraph Co. has joined four other industry firms and several trade groups in still another appeal to the Federal Communications Commission the halt part of a new program under which customers may hooks up their own equipment to the national telephone network.

Even as AT&T subsidiaries around the nation were filing proposed rates with state agencies this week; a letter was sent to new FCC chairman Charles D. Ferris seexing a partial stay of a previous agency order requiring the new program.

The letter, dated Nov. 1 and made available to The Washington Post yesterday, told Ferris that the industry Industry fears "needless and unwarranted confusion among the general public" if the current program remains in effect.

AT&T and the other firms want the FCC to adopt a so-called "primary instrument policy," under which customers would have to retain telephone-company-supplied equipment as their main telephone connected with the national system.

Under new FCC rules which went into effect after being upheld by the Supreme Court, customers may supply all their own equipment and connect the devices to jacks installed by the telephone company.

Without favorable action on their position in the near future, "An irreversible situation is likely to develop," the telephone firms said.

In addition to AT&T, the letter was stuned by officers of United Telecommunications, General Telephone & Electronics, Continental, Telephone Corp., Central Telephone & Utilities Corp., the National Telephone Cooperative Association and other industry groups.