American Telephone & Telegraph Co. poured champagne toasts for its new baby--American Bell--and said that it will do for the data communications business the same thing the Bell System has done for the voice communications field.

American Bell, the unregulated AT&T subsidiary, and its data communications service were unveiled to reporters here yesterday after a champagne brunch, a party the company has wanted to throw for more than a decade.

The service, called Net 1, has been in the planning stages five years. It will offer large and small businesses the ability to send computer data through a common carrier--most likely local telephone companies and AT&T or other long-distance services. AT&T will massage and alter the data to fit the needs of customers.

"At the risk of sounding overly dramatic . . . we at AT&T consider this a major moment in the history of our business," said AT&T Chairman Charles L. Brown.

The service will begin July 1, pending certain Federal Communications Commission approvals and barring an appeals court reversal of the landmark FCC decision that permitted the company to enter new, unregulated businesses.

At an unmarked warehouse near the Hudson River in midtown Manhattan, reporters were briefed on the message and data communications project that bears a striking resemblence to those now offered by General Telephone & Telegraph Co.'s Telenet subsidiary, Tymshare, and several other communications concerns.

AT&T hopes its offspring has some special advantages, notably the almost immediate nationwide scope of the program and the resources of the company, which will have $167 million in initial capital from AT&T, with a planned increase to nearly $700 million within three years.

Net 1 initially is after only a slice of the $7.6 billion data communications market--high-speed specialized services with estimated sales last year of about $160 million. But it is designed to get American Bell into electronic mail and information distribution, a field expected to reach $11 billion in sales by mid-decade.

"This is a vanilla ice cream service," said Howard Anderson, a consultant with The Yankee Group. "What is interesting is when they get to Rocky Road. That will take a lot of market and equipment development. All they've done so far is approach parity with GTE Telenet, the market leader. This will do what Telenet does and probably not as well."

Introduction of the service sets up what is likely to be AT&T's first major confrontation in a generally unregulated environment. It faces competition from GTE, the second largest telephone company; and International Business Machines Corp., through its Satellite Business Systems joint venture.

GTE Telenet, based in Vienna, Va., serves about 700 customers in 250 cities. "They'll get more publicity than we will but they won't be able to deliver very much service because it will take several years to deploy a nationwide network," said Stuart Mathison, GTE Telenet's vice president for corporate planning.