The U.S. Court of Appeals here yesterday took the unusual step of writing a separate opinion to order the Federal Communications Commission and the American Telephone and Telegraph Co. to comply with an earlier appellate court ruling in a hard-fought case involving long distance telephone systems.

The strongly worded order directing compliance basically accused AT&T and the FCC of trying to circumvent the earlier appeals court ruling and "twist(ing) the issues we contemplated in this case beyond recognition . . . "

Written by U.S. Circuit Chief Judge J. Skelly Wright, the three-judge panel said an FCC order in AT&T's favor subsequent to the court ruling "deliberately frustrates the purpose of the litigation, the basis on which it was presented by the parties, and the intended effect of our decree."

At issue in the case is Executent, a service provided by MCI Telecommunications Corp. through which long-distance calls may be placed. With the Executent system, a subscriber with a pushbutton telephone is able to reach any telephone in a distant city served by MCI by dialing a local MCI number followed by an access code and the number in the distant city.

Mci Chairman William McGowan said yesterday that his firm has authority to serve some 30 cities, and that he and another firm could generate monthly revenues of about $10 million from the service compared with AT&T revenues in excess of $3 billion a month.

According to the appeals court opinion yesterday, MCI has met with "almost continuous resistance from AT&T in its efforts to provide communications services." However, the court said, "we had though that this process finally culminated" when it ruled in Execunet's favor last fall.

AT&T had argued during that proceeding that the public interest would be adversely affected because long-distance telephone rates would have to be raised if too much long-distance traffic was taken away from the phone company in densely populated areas by the new system.

The telephone company asked the U.S. Supreme Court to overrule the appeals court ruling last fall, but the high court refused to take the case on Jan. 16.

The appeals court said yesterday that hours after the Supreme Court refused to hear the case, AT&T took a new approach and announced its intention to provide no additional physical connections to its equipment to allow Execunet to use that equipment - in effect blocking the system from further expansion.

In February, the FCC said it basically agreed with AT&T's position and MCI then asked the appeals court to enforce its earlier ruling in the case.

The appeals court made it clear that it thought both the FCC and AT&T were wrong in the new approach, saying the new position "means that MCI is in effect no better off than it was during the entire course of the litigation in this court: notwithstanding our favorable decision, it is unable to expand Execunet."

The court said that "MCI certainly has good cause to feel that this subsequent turn of events engineered by the commission and AT&T is unfair."

It pointed out in the Wright opinion, with which U.S. Circuit Judges Edward A. Tamm and Malcolm R. Wilkey agreed that the issue of new physical interconnections was not raised at all by AT&T in the earlier litigation.

The court went through in detail its reasoning for ruling in Execunet's favor last fall and said "the only conclusion . . . is that" AT&T must provide the interconnections for Execunet. It said the FCC "Acted Inconsistently" with the earlier court ruling, and "AT&T is acting inconsistently with the view of its legal obligations. . ."