International Telephone and Telegraph Corp. has urged the Reagan administration to develop an alternative to the telecommunications legislation passed by the Senate Commerce Committee while simultaneously saying it opposes a possible decision to drop the government's antitrust suit against American Telephone & Telegraph.

"Passage of S.898 [the Senate legislation] or the dismissal of the government's antitrust suit would effectively exempt the world's largest private monopoly from the antitrust laws, despite repeated court decisions holding AT&T guilty of antitrust law violations and AT&T's continuous filing of unlawful tariffs in the face of potential competition," said George Knapp, ITT's vice president for communications companies, in a letter to Commerce Secretary Malcolm Baldrige.

ITT, a worldwide company with $17 billion in sales, operates a domestic microwave communications company in competition with AT&T's long distance network.

In letters sent last week to Baldrige and Bernard Wunder, assistant secretary of Commerce, ITT stressed that in their view the legislation passed 16-to-1 by the Senate committee earlier this month, is a new version of the 1976 "Bell Bill," will "remonopolize" the nation's telecommunications industry, decrease competition, and actually increase the Federal Communications Commission's regulatory role.

The legislation would permit AT&T to enter new markets through separate subsidiaries and is designed to limit transactions between the parent company and its affiliate. ITT called the plan a "facade of separation and independence."

Instead, ITT proposed a different restructuring of AT&T, suggesting the Bell System spin off its new company or companies to AT&T stockholders.