The Federal Communications Commission yesterday delayed implementation of last year's landmark decision deregulating most new telephone equipment along with American Telephone & Telegraph Co.'s new authority to enter unregulated businesses.

The decision was made as the agency's staff admitted to the six-member commission that they would not be prepared to monitor the massive deregulation project, known as the Computer II decision, by the scheduled date of March 1, 1982.The commission delayed implementation of the Computer II decision for 10 months while postponing indefinitely AT&T's authority to engage in unregulated activity.

The FCC also rejected a proposal by AT&T that would have deregulated all telephone equipment on customer premises on March 1. Although the company, in a position backed by Commissioner Joseph Fogarty, argued that the FCC action would lead to massive confusion, the FCC insisted that only new equipment be deregulated.

In addition, the commission affirmed an earlier, controversial decision to raise AT&T's regulated rate of return on investment from 10.5 percent to 12.75 percent. The decision was made over the objections of two members of Congress, Reps. Edward Markey (D-Mass.) and Ronald Mottl (D-Ohio), who had asked the FCC to reconsider the action.

And in another decision with broad impact for the Bell System, the FCC voted to deny an AT&T request that a new series of calling services be operated in an unregulated environment under the new competitive subsidiary mandated by Computer II.

The series of actions marked the revamped FCC's first major telephone industry decisions. The FCC is now run by a Republican majority and is chaired by a Reagan appointee, Mark Fowler, a broadcasting lawyer with little previous experience in common carrier matters.

The actions were sharply criticized by a top AT&T official. "We realize that all the regulatory tasks could not be completed in time for the FCC to meet the March 1, 1982 date," said James R. Billingsley, AT&T's vice president. "We can live with a nine-month delay.

"But we think the FCC's new method for detariffing telephone sets and customer equipment is all wrong and will result in confusion and higher costs for our customers. The FCC's actions today don't solve the problems of implementing Computer Inquiry II. By ducking the issues it has actually compounded the problems."

In another action, the FCC delayed the effective date of its so-called cellular radio systems. The commission had planned to begin accepting applications on Nov. 23, but instead said it would not take these applications until 30 days after finishing a reconsideration order, an action expected in December.