Law enforcement authorities here are investigating allegations that a group of American University students routinely billed thousands of dollars of long-distance telephone calls to the private WATS line of an Arizona congressman.

Students allegedly made most of the calls from their dormitory rooms over the last four to six months to friends and relatives from Maine to California as well as frequently calling a New York number for a recording of sports scores.

Three American University students have been ordered to appear in the U.S. Attorney's office next week for a hearing to determine if criminal prosecution is warranted in their cases. Meanwhile, investigators have said that students at campuses throughout the Washington area -- particularly at Georgetown and George Washington universities -- also may have used the telephone line charged to Rep. Bob Stump (D-Ariz.).

"I don't think it's comical at all," said an annoyed Stump, who added that he considers the students' use of his telephone line to be fraud. "There's not much difference between what they were doing and some of this Abscam stuff," Stump said in a telephone interview.

As recently as 11:30 on Monday night, Stump said he watched in amazement as the light on his Capitol Hill office telephone, which indicates the WATS line is in use, blinked on and off as many as 40 times as the students gabbed away. The students were able to us Stump's telephone line by dialing in access number and then holding down two coded numbers at the same time.

Aside from the constant unauthorized use of his telephone, Stump said he is particularly upset because with the access code the students possessed, they might have listened in on his private phone conversations.

Stump said in a telephone interview that the recent rash of calls on his office WATS line was the second time he has had to cope with unauthorized callers. About a year ago, the congressman said he was forced to change his WATS access code after hearing callers brag on his line, "Hey, this is a freebie." Stump said that at times he has told people, "Hey, get off the line!" and they ignored him.

Chesapeake & Potomac Telephone Co. officials said they do not know how many calls were made on Stump's line or how much they cost. Ultimately, honest telephone users may have to absorb the cost of the fraudlent calls through higher rates, the officials said.

"We're not interested in putting kids in the slammer. We'd rather they didn't do it," said Clayton Brown, a security investigator for the Chesapeake and Potomac Telephone Co., which estimates it loses $100,000 a year here as a result of long distance fraud, most of which occurs on college campuses. "There's is no such thing as a free call," Brown said.

Investigators said they have not been able to determine how the students obtained the access code they needed to use Stump's Capitol Hill Wide Area Telecommunications Service (WATS) line, a special bulk-rate telephone service available to high-volume long-distance call users.Stump has told investigators that no American University students have worked in his office in the past two years and that only he and a high-ranking aide knew the code in his office. Many of the students were not aware that the code they had belonged to a congressman, the investigators said some students told them.

Stump complained to Capitol Police about two weeks ago that unauthorized persons were tying up his WATS line, investigators said. Officials traced the calls back to American University with the use of sophisticated electronic tracking equipment and after hearing bits and pieces of various conversations, including a few first names and some chatter about an intramural football game.

On Wednesday, investigators rounded up more than a dozen students from an AU dormitory room at Letts Hall just after several long distance calls were made on Stump's WATS line from a telephone there. The students were quietly taken to the nearby campus security office where they were questioned by District police Lt. Richard F. Xander, who is supervising the investigation for the Capitol Police. Three students were eventually summoned to appear in the prosecutor's office at 9 a.m. next Thursday.

I think the phone calls to Mama on legitimate lines were quite heavy" afterwards, said AU security investigator Steve Gorney, who is working on the case with another AU investigator. Arthur L. Henick.

Panic erupted in the blue cinderblock corridors on the third floor of Letts Hall after the investigators appeared there Wednesday afternoon, students said yesterday.

They'd have to take out half the schoof if they had a full-scale investigation," said sophomore Rick Passales, 19. He said he had not used the WATS line.

On Wednesday night, some students, fearful they might be implicated in the investigation, immediately called family lawyers, while others joked about "going to Sing Sing," said student Jay Welfeld, 20.

The students could face false pretense charges, a misdemeanor carrying a penalty of up to a year in jail, a $1,000 fine or both.

Various WATS line codes, not just the congressman's, were passed around the university just after the school year's first telephone bills went out in early October, according to student Chris Hiddleston, 20. Both Welfeld and Hiddleston said they did not use the WATS line. Some students had bills as high as $160, said Hiddleston, bemoaning the cost of long distance calls.

"Where's the money for beer going to come from after that?" he asked. "Or books?"