The Federal Communications Commission is expected to rule next week that local telephone companies should randomly assign millions of customers to a primary long-distance company if the customers do not make the choice within six months of "equal access" service.

"It's going to stimulate competition in the long-distance market," said one FCC official who asked not to be identified.

Equal access, mandated by the breakup of the Bell System, eliminates lengthy codes consumers previously had to dial to use the long-distance systems of AT&T's competitors. The process also gives AT&T's rivals the same quality of connection to the local network that AT&T enjoys.

The $2 billion process of converting local phone companies' switching systems is expected to be completed by September 1986, with 70 million out of a total 111.4 million lines converted to the service, according to the United States Telephone Association (USTA).

Currently, local telephone companies notify customers of long-distance options, but those who do not make a choice among AT&T and its rivals within six months are automatically assigned by "default" to AT&T by the local telephone company. The process has been hotly contested by long-distance companies such as MCI Communications Corp., GTE Sprint, Satellite Business Systems and others as anti-competitive because of the high percentage of consumers who are assigned to AT&T.

Several months ago, Northwestern Bell Telephone Co., a local telephone company owned by regional phone company U S West Inc., became the first to send customers voting ballots.

The idea has struck the FCC as a good way to encourage competition, and the commission may rule to adopt the plan next week. "We would probably follow closely what Northwestern Bell has done," said the FCC official.

Under the plan, customers are sent a ballot 90 days in advance of service and asked to choose a long-distance company. Another ballot is mailed to those who do not choose within six months, informing customers they will be assigned to a company if they do not make a choice.

Once randomly assigned, the customer can switch to another company initially for free. Later on, a small fee is charged for switching. The USTA estimates providing balloting would cost between $1 and $5 per telephone line. The cost ultimately would be borne by consumers, but "would not amount to very much," the FCC official said.

AT&T has fought the proposal. "Our view has been that while we feel people should make a choice, we feel if they don't, it's not fair someone else should make that decision for them," said Edith Herman, a spokeswoman for AT&T. "We also feel people who don't make a choice are making a choice for AT&T."

Nevertheless, consumer advocates and some local phone companies view balloting as a necessary step in making a successful transition to competition.

"We do not support the current default procedure," said Samuel A. Simon, executive director of the Telecommunications Research & Action Center, a Washington consumer group. "It is anti-competitive and a paternalistic approach -- if people don't act, why are we giving them something at all?"

Other regional telephone companies are considering balloting, among them Nynex, Bell South and Bell Atlantic Corp., according to the USTA.