Now begins the time for decision for AT&T's many owners

Now that American Telephone & Telegraph has disclosed its estimate of how its spun-off parts will fare, shareholders can begin to decide which stocks to keep and which to sell.

But investors can only begin to decide.

Even though the new stock is to start trading next Monday on a when-issued basis, it will be some weeks before all 3.2 million shareholders receive their prospectuses. Those documents, condensed from a massive, 20,000-page AT&T filing with the Securities and Exchange Commission Nov. 16, are providing investors and their financial advisers with a glut of information.

That information is only a highly hedged conjecture of how the new, smaller AT&T and the six new regional phone companies will do after the telecommunications giant splits up Jan. 1. For each share of AT&T stock owned, shareholders will receive 10 shares of the new AT&T and one share each of the new regional phone companies.

No one is really sure how the stocks of the eight new companies will do, since there is no track record for these companies, only for their parent. The first real evidence will not be available until May 1984.

At this point, moreover, assets have not yet been fully divested; this will be completed Jan. 1. Possible federal government regulations over access charges may influence earnings. And competition from other telecommunications firms - from General Telephone & Electric to MCI Communications to IBM - could take its toll, especially on the new AT&T.

AT&T will have to compete with firms that are implementing new technology that can obviate some of AT&T's costly hardware. Land-based long-distance lines could become obsolete and smaller high-tech firms might out-innovate AT&T's Western Electric subsidiary in telecommunications hardware.

AT&T's own projection puts the performance of six regional phone companies close to even. Bell South would have the highest first-quarter dividend ($1.95); Pacific Telesis and US West the lowest ($1.35).

Most analysts see Bell South and Southwestern Bell - both in population growth markets and relatively lax regulatory climates - as in the strongest earning positions. NyNex, in the highly important information corridor centered on New York City, also appears favorable. Bell Atlantic appears to offer market stability.

Chicago-based Ameri-tech, however, is believed to suffer somewhat from the sluggish economy of the upper Midwest. Pacific Telesis is in a good economic setting but experiences the heaviest state and local government regulation. US West covers the greatest land area but has a small population and much rural service to maintain.

With all of these variables thrust upon them, investors - especially many of the rentiers who traditionally hold AT&T stock - undoubtedly will have questions. For a fee (usually a percentage of the AT&T stock) brokers are offering their advisory services to sort things out. A number of financial packages are being offered by these firms.

''It is a very, very complex subject,'' says John Healy of Sanford C. Bernstein & Co. ''Professional advice is important.''

Bernstein and most other brokerage houses feel fairly confident that AT&T and the six regional phone companies are good investments. A comprehensive Bernstein report says: ''The confusion generated by the divestiture, coupled with AT&T's poor current earnings performance, has caused the market to underestimate earnings growth over the next 3 to 5 years and overestimate the inherent level of risk.''

Using 1983 financial information, Salomon Brothers analyst Mark D. Luftig projects the sum of post-divestiture stock at $64 to $68 a share. AT&T closed Nov. 16 at $63 1/8 up 7/8. He says he was not especially surprised by AT&T's own estimates.

Merrill Lynch analyst Leonard S. Hyman says the AT&T estimates are ''very close to what we have been carrying.'' Investors, says Mr. Hyman, now have great latitude in choosing how to hold phone stock. They may want to concentrate solely on the new AT&T for potential capital appreciation or on the highest yielding regional stocks for income purposes or various combinations.

AT&T's first-quarter performance estimates (After Jan. 1 divestiture)m

Revenue Income Dividends (billions) (billions)m AT&T $56.54 $210.000 $.30 Ameritech 8.34 .923 1.50 Bell Atlantic 8.32 .952 1.60 BellSouth 9.80 1.200 1.95 Nynex 9.83 .937 1.50 Pac Telesis 8.08 .827 1.35 SW Bell 7.75 .869 1.40 US West 7.44 .877 1.35 * Estimated May 1984 quarterly dividend for stockholder is calculated by adding AT&T dividend (30 cents) to 1/10 of the dividend of operating companies. This equals $1.365.