American Telephone & Telegraph Co. said yesterday it plans to ask for a stay of last week's court decision that AT&T must make available the additional phone lines necessary for MCI Communications Corp. to expand its Excunet business service.

James E. Olson, executive vice president at AT&T, said, "We intend to review legal alternatives open to us." Meanwhile, Bell System operating companies will immediately begin processing requests by MCI for the additional lines, as ordered by the U.S. Court of Appeals here.

If the stay is denied, Olson said, AT&T will file rates with the Federal Communications Commission.

The court last July overturned an FCC ruling that Execunet should be halted because it is now a private line business service but similar to Bell System's regular long distance and Wide Area Toll Service.

AT&T refused to supply additional lines unless MCI could show it was in the public interest. The FCC upheld that view on Feb. 12 and MCI appealed to the court.

MCI's chief executive officer, William G. McGowan has accused AT&T of distorting the impact that competition has had on the company.

"Last year, after eight years of vigorous incursions on AT&T's monopoly preserve by the MCIs of the world," McGowan said, "AT&T reported revenues of $36 billion - more than double those of 1969 - and profits of $4.5 billion - again exactly twice 1969's results."