LEAD: Japan's dominant phone company, under pressure from the Government, said Friday that it would disclose market information that would help its new long-distance competitors plan their strategies.

Investors fear not only price wars but also new long-distance competitors, especially the regional Baby Bell companies.

But another long-distance competitor that was a party to the negotiations, Sprint, supported the deal.

The company has said it may leave markets or reduce services in response to a court decision that allows the Bell companies to charge long-distance competitors higher fees to use their local networks.

Meanwhile, Sprint's long-distance competitors, AT&T and MCI, were making their move into the Latino market.

Mexican regulators have not yet set out all the criteria for selecting long-distance competitors.

They attribute their long-distance competitors' hardships to less-than-expert management and inexperience with the Mexican market.

All its long-distance competitors would have had to pay fees, based on time and distance, for the use of local telephone lines to the interconnection points.

Still, consumer advocates argue that as upstart local telephone companies and long-distance competitors have dried up or been acquired, the Bell and cable companies have become ever more powerful.

Lucent's overtures are also being favorably received by AT&T's long-distance competitors.