In a move that would sharply expand both its local telephone operations and its fast-growing cellular telephone business, the [GTE Corporation] said yesterday that it had reached a preliminary agreement with the Contel Corporation to acquire the Atlanta-based company in a stock swap. The deal would be worth about $6 billion, based on yesterday's closing stock prices.

If the deal is completed and approved by regulatory authorities, GTE, based in Stamford, Conn., would become the fifth-largest local telephone company in the nation, based on number of telephone lines - after the Bell Atlantic Corporation, the BellSouth Corporation, Ameritech and the Nynex Corporation - and its second-largest mobile phone operator, after McCaw Cellular Communications.

''Aside from the obvious synergy of our telephone and cellular operations, both companies have other areas of interest that are remarkably parallel,'' James L. Johnson, GTE's chairman and chief executive, said in a statement. He cited businesses that supply communications systems to governments, satellite communications operations and aggressive programs by both companies to gain a foothold in cable television services.

''It's a great strategic move and very gutsy,'' said Jack Grubman, who follows the telecommunications industry for Paine Webber. ''The bigger you are in this industry, the better off you are. They got the best of the independent companies at a price that will look good in three or four years.''

Investors took a shorter-term view, however. GTE's stock price fell $1.25 a share, to $29.75, in trading yesterday on the New York Stock Exchange. The more than 200 million shares GTE will issue to complete the transaction at an exchange rate of 1.27 GTE shares for each Contel share could dilute GTE's share earnings this year by about 10 percent, analysts said.

Contel shares rose $7.125 to finish at $35.125 a share on the New York Stock Exchange. Charles Wohlsetter, the entrepreneurial 80-year-old chairman of Contel, said that because of GTE's higher stock dividend, Contel shareholders will receive the equivalent of $1.85 a share in dividends for their stock after the deal, a sharp increase from Contel's $1.10-a-share payout.

The proposed deal is one of the largest to date in the telecommunications industry, which has been undergoing a period of consolidation. The MCI Communications Corporation recently bought Telecom USA Inc. for $1.25 billion. In April, GTE agreed to acquire the cellular phone operations of the Providence Journal Company for $710 million, and United Telecommunications Inc. said it would pay $500 million to complete its buyout of GTE's interest in the U S Sprint Communications Company. U S Sprint is the nation's third-largest long-distance enterprise, after the American Telephone and Telegraph Company and MCI.

Earlier this year, McCaw secured its place as the nation's largest cellular concern, based on number of potential customers in its service areas, by paying $3.4 billion for control of the Lin Broadcasting Corporation.

Mr. Wohlsetter and John L. Segall, Contel's vice chairman, will become vice chairmen of GTE and members of its board if the deal is completed. Mr. Wohlsetter would also become chairman of GTE's strategic issues, planning and technology committee. However, the actual roles of both men and of Donald M. Weber, Contel's president and chief executive, at GTE remain to be worked out, said Harvey W. Greisman, a GTE spokesman.

$3.1 Billion is Sales

Contel's attractions for GTE include 2.6 million telephone lines in 30 states, with a far-higher-than-average percentage of its exchanges employing modern digital switches. And Contel has completed a reorganization in which it shed a number of money-losing businesses accumulated during an ill-fated period of diversification. It netted $277 million on revenues of $3.1 billion last year.

Contel Cellular Inc., which is 90 percent owned by the Contel Corporation and 10 percent publicly owned, has franchises in 36 metropolitan areas. The franchises include valuable regions in the Southeast acquired last fall from McCaw for $1.3 billion, making Contel Cellular the seventh-largest cellular business in the nation.

GTE has undergone a wave of restructuring since Mr. Johnson became chief executive in 1988. In addition to phasing out GTE's ownership interest in U S Sprint, Mr. Johnson has shifted headquarters for the local telephone operations to Dallas, begun a drive to reduce employment by 14,000 and reduced GTE's involvement in the development and manufacturing of telephone switches by setting up a joint venture with A.T. &T.

GTE employs 158,000 people and has businesses in 46 states and 41 countries, including local phone companies in Canada and the Dominican Republic that serve 2.3 million customers. In addition to telecommunications, it has a large lighting business. It had net earnings of $1.4 billion on sales of $17.42 billion last year.

Comsat, struggling to recover the growth rates it grew accustomed to in the early days of communications satellites, said yesterday that it would merge with the Contel Corporation in an exchange of stock worth about $2.4 billion.

Under the agreement, Comsat, formally the Communications Satellite Corporation, will retain its name and pursue its unique role as the Federal Government's chief vehicle for operating communications satellites in cooperation with foreign nations.

But control of the company will go to top executives at Contel, which is attempting to speed its entry into the unregulated portions of the telecommunications business - including internal communications for the nation's largest companies and sales of advanced telecommunications equipment to the Federal Government. Contel's primary business is the operation of local telephone companies in 30 states.

Mutual Benefit

The merger will give the Atlanta-based Contel, whose $2.5 billion in revenues last year made it five times Comsat's size, access to Comsat's highly regarded technologies. Contel would also benefit from its role as the international satellite carrier for everything from telephone companies to television networks.

Meanwhile, the Washington-based Comsat will gain the financial resources and marketing power that it has failed to muster in recent efforts to diversify into a range of other communications ventures.

''We realized that we simply did not have a market position in the competitive side of our business,'' said Irving Goldstein, Comsat's chief executive, who will become vice chairman of the combined venture. ''I think now we will have the staying power, the marketing clout and the technical capability that neither company alone had enough of.''

Terms of Agreement

Under the terms of the agreement holders of Contel's 76 million outstanding shares will each receive 94-hundredths of a share of Comsat common stock for each of their Contel shares. Comsat closed Monday at $34.25, meaning that Contel shareholders will receive the equivalent of about $32.195. Comsat also said its annual dividend would rise to $2 from $1.20.

Analysts noted that the merger -which was approved by the boards of both companies last Friday, but still requires a go-ahead from the Federal Communications Commission -marked an acknowledgement that Comsat was facing increasing competition in the business where it once held a monopoly: establishing satellite communications between the United States and virtually every other country in the world.

That was Comsat's charge when it was created in 1962 by the United State Communications Satellite Act. At the dawn of the space age, Comsat's chief task was setting up Intelsat, an international consortium responsible for the construction, launching and operation of communications satellites for more than 100 countries. Later, it started Inmarsat, which established the first satellite links to ships and oil platforms at sea. A Hot Stock in 1964

When it went public in 1964, Comsat was considered the hottest of high-tech stocks, with investors receiving only about three shares for every 100 they ordered. Even at the time, many thought the company was racing a bit ahead of the technology. Philip L. Graham, the late publisher of The Washington Post and an early member of the Comsat board, said creating the company was ''a little like trying to create an international airline six months after Kitty Hawk.''

The company's first satellite, Early Bird, increased the number of international circuits by 50 percent and permitted the first live television transmissions from abroad. And through the early 1970's, the company grew by leaps and bounds - until it became clear that demand for telecommunications was not growing as fast as capacity. Ill-Fated Diversifications

That realization, along with the entry of RCA and other competitors into the international satellite business, led Comsat into a host of telecommunications ventures. Most notable among them were the ill-fated Satellite Business Systems, a partnership with the International Business Machines Corporation and Aetna Insurance to provide communications services to large corporations, and a pay-TV spinoff that was intended to beam television programming from satellites directly into homes.

Both ventures ran up tens of millions of dollars in losses, and were dumped in a restructuring led by Mr. Goldstein and Marcel P. Joseph, Comsat's president and chief operating officer, a longtime veteran of General Electric who is regarded as a tough-minded engineer. Both also worked extensively with Contel, formerly Continental Telecom Inc., when the two companies built a communications system recently for Halliburton Company.

''That led us to discussions of more and more joint ventures,'' said Mr. Goldstein. ''Then in the past month the idea emerged - I can't really remember by who - that a merger would really be the best solution.''

The structure of the deal was influenced by Comsat's Congressional charter. ''No matter what, Comsat had to be the surviving entity,'' said Susan O'Connell, special assistant to the chief of the F.C.C.'s Common Carrier Bureau.

At Contel yesterday, executives welcomed the merger. ''Having international scope is important to our future, and the Comsat name will help us toward that end,'' said John N. Lemasters, Contel's 52-year-old chief executive, who will take control of Comsat's executive suite along with Contel chairman Charles Wohlstetter. Contel's American Satellite subsidiary, he said, ''has the network we need, and Comsat has the customer base.''