MCI Communications Corp., planning for expansion as the break-up of American Telephone & Telegraph Co. nears, said yesterday it is boosting its planned capital expenditures for the coming fiscal year to $1 billion from the current year's level of $600 million.

The company said it will spend the money on equipment to expand and upgrade its long-distance voice network and to expand its efforts in digital transmission of data in the fiscal year beginning April 1.

The Washington-based concern, which already has announced plans to spend hundreds of millions of dollars over the next few years on fiber-optics transmission systems and satellite transponders, said the $1 billion will be earmarked for those projects, for purchase of digital radio transmission equipment and additional fiber-optics and switching equipment, and to expand MCI's existing microwave network.

An MCI spokesman declined to give a detailed breakdown of how the money would be spent, but said that most of the $200 million to $300 million cost of the satellite transponders would be included in the capital budget and that the company also hopes to complete a good part of its fiber-optics network along the Amtrak right-of-way on the East Coast during the coming fiscal year.