A telephone company that charges to provide unlisted numbers now wants to sell another service that gives customers a peek at the number of the person calling before they pick up the phone.

The "Caller ID" service, already used in New Jersey and proposed in several other states, is drawing static from consumer and civil rights advocates who say it would invade privacy and break anti-wiretap laws.The state Public Utility Commission, which meets Wednesday, must approve Bell of Pennsylvania's proposal before it takes effect. The company, a subsidiary of Philadelphia-based Bell Atlantic Corp., says it hopes to initiate the service by March 19.

Caller ID would enable customers to view on a display device the telephone number of the caller's phone, even if that number is unlisted. The device would cost about $70 and the service $6.50 monthly.

Bell of Pennsylvania says the service would enhance the privacy of the person being called, enabling him or her to screen incoming calls and pinpoint their origin.

"It's much like a peephole in a door in your home. It allows you to see who's at your door," company spokesman Miles Kotay said.

The company has invested about $20 million in Philadelphia alone to upgrade its equipment to handle a range of services including Caller ID. Kotay said the company expects revenues "in the tens of millions of dollars" from the services within five years.

But state Consumer Advocate David Barasch warned the service could be devastating, for example, to someone staying in a shelter for battered women who needs to contact her husband but would not want him to know the shelter's number.

"Once somebody has your number, it doesn't take a lot of effort to find out where you are," Barasch said.

Barry Steinhardt, executive director of the American Civil Liberties Union in Pennsylvania, argues the service would be illegal.

Steinhardt filed a complaint Friday with the Public Utility Commission contending the state Wiretapping and Electronic Surveillance Act strictly limits the use of "trap and trace devices" that identify the number of a phone on which a call is being placed.

"Bell's customers would all be committing a third-degree felony if they ordered this," Steinhardt said.

Kotay disputed the question of illegality, saying the company had concluded after a "very careful examination of the laws" that Caller ID would be legal. He declined further comment pending action on Steinhardt's complaint.

The introduction of such service is likely to be proposed around the country as companies upgrade their switching and signaling equipment to handle the sophisticated information it requires.

Bell Atlantic subsidiary New Jersey Bell began offering Caller ID late last year after a six-month trial period in two areas of the state serving 213,000 residential customers and 85,000 business customers, said George Dawson, a spokesman for the New Jersey Board of Public Utilities.

Fears that the service would be used by businesses to obtain numbers for telephone solicitations did not pan out, he said. Overall, the service drew positive support, he said.

Since the introduction statewide, the Board of Public Utilities had received about 50 complaints, mostly from people with unlisted numbers, Dawson said. The rate of complaints had dropped to about two or three a month since the first weeks of the service, he said.