The Federal Communications Commission yesterday rejected an industry proposal to require telephone users to use at least one instrument owned and maintained by the telephone company.

In a unanimous vote, the commission concluded that the "primary instrument" proposal was fundamentally inconsistent with the principles of past FCC decisions that opened the way for competition in the supply of telephone equipment.

The telephone industry came up with the idea after the Supreme Court last year declined to review an FCC program that allows telephone users to connect their own terminal equipment-main and extension telephones as well as other kinds of phone systems - to the national telephone network without carrier-supplied protective devices, so long as the equipment complied with FCC standards to protect the network from harm.

Yesterday, the commission again rejected on oft-repeated industry claim that a carrier instrument was an integral part of complete telephone service. The FCC said it found no showing of public detriment that would warrant a restriction on the right of the single-line subscriber - those with one line coming in to a home or business-furnish their own primary instrument.

In another action, the FCC indicated it would seek to expand the types of services that American Telephone & Telegraph Co. can offer its customers.

The commission tentatively voiced support for staff proposals that would, among other things, start discussions with the Justice Department to change a consent decree that bars AT&T from offering services that may invoice data processing.