As Montgomery County Police Officer Ronald J. Gaddis faced a disciplinary hearing last October, county officials told him that a tape recording of a telephone conversation between Gaddis and a police dispatcher would be used as evidence against him on a charge of orally abusing the dispatcher.

Gaddis was the latest of four police employes to be confronted with tapes of nonemergency phone calls, which the department sought to use in disciplinary actions, according to Walter Bader, president of the Fraternal Order of Police.

Now Gaddis, along with 11 fellow officers and the police union, is suing the department for $865,000 over its practice of taping nonemergency telephone calls, claiming it is a violation of their rights, according to a lawsuit filed this month in Montgomery County Circuit Court.

The suit contends that the police taping system is illegal under state law, which allows taping of nonemergency calls only with the consent of both parties to a conversation.

Two-party consent is not required when employes of police emergency communication centers are answering emergency calls, according to the state law.

The police department regularly tapes both its 911 emergency calls and the calls on its main number of 279-8000 because both numbers feed into the telephones at the Operation Emergency Center in Rockville, according to police officials. The same system also tapes radio traffic between police cars and dispatchers.

The department believes it useful to record both lines because citizens often use the 279-8000 number to report situations that are actually emergencies, according to Bernard Keller, a dispatch supervisor. The recordings are useful for police to double-check information, especially if the call is inadvertently disconnected, he said.

Callers to the nonemergency number are warned by operators that their conversations are being recorded, according to Keller. Police operators routinely answer the phones by saying, "All lines are being recorded."

Recording telephone conversations at police emergency communications centers is done in many police jurisdictions in this area. Some police departments use a beep in the background to alert callers that their conversations are being recorded.

Bader maintains that even a beep does not constitute consent and that without two-party consent, recording is a felony under state law. He also contended that it is not necessary to record the nonemergency lines and that department officials are "making excuses for their own oversight" for putting in a system that tapes off telephones rather than specific numbers.

Bader, whose union represents officers below the rank of sergeant, said the department should alter its practice and record only the 911 line and radio messages. Police officials would not comment on the substance of the union's lawsuit.

In Gaddis' case, Bader said the officer was alleged to have used "inappropriate" language in a conversation and the tape was going to be used against him in an internal affairs hearing last October.

The union contended that the tape, which was the only piece of evidence against him, was illegal because Gaddis did not consent to the taping, so it should not be admissible as evidence, according to Bader, who is also a plaintiff.

While Gaddis and his attorney filed to have the tape suppressed, Gaddis, an officer for more than 10 years, retired on a disability unrelated to the internal investigation charges, according to Allen J. Katz, the plaintiffs' lawyer. Gaddis was not available for comment.

Although there have been only four cases in the last 10 years in which the department threatened to use tapes in disciplinary actions, Bader said, "Once is too many. A violation of someone's rights is inexcusable."

In a 1982 incident, a dispatcher's conversation off-duty on a nonemergency line was taped and introduced as evidence of the "volatile nature" of the employe at a merit board hearing, according to the dispatcher, who asked not to be identified.

The veteran dispatcher said very few calls recorded on the nonemergency lines turned out to be emergencies. "Never have the tapes been used for the benefit . . . of the citizen or the dispatcher," she said.

Keller said police usually keep the tapes for 30 days before recording over them. But Bader said police routinely put tapes on hold for an indefinite period to use for internal investigations.