NEWARK, N.J. (AP) _ A service that would tell New Jersey Bell customers who is calling before they answer the telephone represents an invasion of privacy that also could reduce calls to emergency hot lines relying on anonymous tips, opponents told a regulatory panel Monday.

The state Board of Public Utilities is considering the proposed Call Identification service, described by New Jersey Bell as a way for customers to ''see who is at the door before opening it.''

Although opposed by New Jersey's public advocate and American Civil Liberties Union chapter, the service has the support of law enforcement officials who see it as a way to thwart obscene callers, reduce sales solicitation calls and assist in the tracing of numbers during emergencies.

Call Identification, which New Jersey Bell said now is offered only in Orlando, Fla., displays the telephone numbers of incoming calls on a small screen hooked to the phone. New Jersey Bell wants to test market the service and six other features in Atlantic City and the New York suburbs of Hudson County.

Raymond Makul, director of the public advocate's Division of Rate Counsel, told the utilities board that the benefits of Call Identification do not ''exceed the related potential for public harm.''

Approval of the service would mean that ''we are being looked out for by our Big Brother. Big Brother Bell,'' Makul said.

''Call Identification must be seen as a new toy for the over-zealous bureacrat, the over-ambitious salesperson and over-inquistive neighbor,'' Edward Martone, executive director of the ACLU in New Jersey, said in a letter presented to the utilities board.

Opponents said the service would violate the agreement between New Jersey Bell and customers who paid extra for unpublished numbers. Martone said he would consider filing a class-action suit on the customers' behalf.

Those opposed also said people giving anonymous tips or getting information from referral hot lines might unwittingly reveal their identities.

For example, Makul said, customers might be reluctant to participate in New Jersey's tax amnesty program or get birth control information if they thought confidentiality was not assured.

James Louis, the state's deputy public defender and director of a program that handles child abuse and neglect cases in family court, said the service would have a ''chilling effect'' on child abuse reports.

But the superintendent of the state police, Col. Clinton A. Pagano, said he sees Call Identification as ''welcome relief.''

The utilities board could rule within a week on New Jersey Bell's request for a two-year test. The telephone company conducted a sixth-month experiment at an Atlantic City casino hotel, where officials said the test was a success but the service was not needed.

If the board grants the trial period, opponents can either supply the board with new information against the project and ask the board to reconsider or appeal the decision to the appelate divsion of the state Superior Court.

Proposed rates for Call Identification are $6.50 per month for residential customers and $8.50 per month for businesses. The company hopes to offer Call Identification and the other services to customers across the state by 1989.

The other services would allow subscribers to block calls from certain numbers, retain the last number from which a call was dialed, repeatedly dial numbers, use tones to distinquish callers, prioritize forwarded calls and allow tracing of the last call received for a fee.