The impending breakup of the Bell System helped MCI Communications Corp. report record financial gains yesterday for the fiscal year ended March 31, 1983.

The Washington-based company earned $170.8 million ($1.69 a share) last year, up 98 percent from $86.5 million (91 cents) the year before.

MCI exceeded $1 billion in sales last year for the first time in its 15-year history. Total fiscal 1983 sales were $1.1 billion, up 112 percent from $506 million in fiscal 1982.

MCI Chairman William G. McGowan said the terms of the court-ordered breakup of American Telephone & Telegraph Corp. contributed to his company's performance last year.

The breakup isn't effective until Jan. 1, 1984. But conditions set forth in the divestiture order have allowed MCI to extend its long-distance services to Canada and to take advantage of long-distance markets within state lines, McGowan said.

MCI had 1.2 million customers in fiscal 1983, up from 739,000 in the previous year.

McGowan said yesterday that his company will increase its customer base again in fiscal 1984, largely because the divestiture allows rotary phone access to MCI's network.

Rotary phones account for 55 percent to 60 percent of all phones in use today in the United States. But rotary phone signals must be converted to touch-tone signals to be used by computerized phone networks, such as that operated by MCI.

Under the current system, AT&T, which controls domestic long-distance lines, has the exclusive right to convert rotary to tone signals for users, such as MCI. But, for what AT&T called justifiable practical considerations and for what MCI called an anticompetitive plot, the signals were never converted for MCI.

The divestiture order, which strips AT&T of its 22 local operating companies, allows equal access to Ma Bell's long lines.

In over-the-counter trading yesterday, MCI closed at 22 5/8 bid, down 3/8 on volume of 1,200 shares.