WASHINGTON — A growing number of software companies are trying to take the Internet back to the future by offering a crude form of two-way voice communication on the popular worldwide computer network at a fraction of the cost of a regular long-distance telephone call.

The mushrooming popularity of the communications software--which requires a powerful computer with an Internet connection, microphone and sound card--poses a threat to long-distance carriers, which are already facing the prospect of new competition from cable operators and local phone companies as a result of telecommunications reform legislation signed into law last month.

More ominously, the software has federal regulators mulling the financial impact of yet another Information Age technology that allows consumers and businesses to bypass the phone network and avoid paying local phone-access charges. On Monday, for instance, a national trade association representing local telephone carriers, filed a petition requesting that the Federal Communications Commission "immediately stop" Internet phone software from facilitating free long-distance calls over the Internet.

In an 11-page petition, America's Carriers Telecommunications Assn. asked the FCC to declare its authority over telecommunications services that use the Internet, telling the agency: "it is not in the public interest to permit long-distance service to be given away, depriving those who must maintain the telecommunications infrastructure of the revenue to do so."

Those revenues, which total $20 billion a year, subsidize telephone service in high-cost areas.

Although only about 20,000 people are now regular users of Internet phone software, one Internet expert estimated that at least 500,000 copies of such software were distributed last year and that as many as 4 million copies could be in circulation by the end of 1997. One major phone user, Cornell University, said it is seriously considering diverting some of its voice-telephone traffic to its campuswide computer network, a move that could eventually pave the way for the school to shift all of its long-distance telephone calls to the Internet.

"This has the potential to grow into a multibillion-dollar business," said Jeffrey Pulver, who compiled the software estimate and who publishes NetWatch, an Internet site that keeps abreast of trends on the global network. "This technology will revolutionize the way we use telephones."