The Federal Communications Commission voted yesterday to investigate the possibility of forcing American Telephone & Telegraph Co. to offer its long distance phone service for resale and shared use.

The FCC's 4-to-3 decision, issued in response to a petition from Washington-based MCL Communications Corp., lays the groundwork for the possibility of telephone customers sharing or reselling services, such as Wide Area Telephone Service (WATS) with other businesses. For instance, MCI could provide its own WATS service, using AT & T lines and paying for the use.

The inquiry also involves AT & T's basic long distance service and its 800 network.

Under current commission policy, such resales and line sharing are prohibited by existing tariff or rate structures. The FCC emphasized that the opening of the inquiry would not affect efforts by firms other than AT & T to develop similar long distance programs.

The commission, stressing that the decision is part of its ongoing program designed to open the industry to competition, said the decision ultimately could provide the "opportunity for market operations rather than discretionary authority on the part of a single firm" to guide the telecommunications market.

Although the FCC is conducting a broader study of all private line services offered by AT & T, the commission staff recommended the new study in light of the MCI petition.

FCC member Joseph R. Fogarty, in a dissent also signed by Commissioners Anne P. Jones and James H. Quello, called the new inquiry "duplicative" and "prejudicial to the other year-and-a-half old investigation.

Although the FCC announcement is designed only to seek public comment on the proposal, the FCC dissenters called the decision "naive and short-sighted," and said the two inquiries cannot be concluded independentlyof of each other."

AT & T said the company was "surprised" by the FCC move, in light of the ongoing review of the telephone industry's market structure.

The company spokesman also said that if the commission endorsed an experiment or the sharing and resale program, another alternative proposed by MCI, AT&T would be willing to set up a "limited trial of WATS resale and sharing after restructed WATS rates are filed" with the commission.

The AT&T reaction indicates that large-scale users of their services, such as the Air Transport Association of America, might face a WATS rate increase if the proposal is enacted. e so-called "back alley garage," as ragsdale put it, that causes problem