It seemed like harmless fun when Virginia Boyle's 10-year-old twins asked if they could call "Santa Line," a tape-recorded telephone message from old St. Nick.

But it was no laughing matter, Boyle said, when she discovered that the "Ho, Ho, Hos" she thought the children were listening to, actually were the breathy pants of a woman describing an explicit sex act.

As it turned out, only one digit separated Santa's number from a new pornographic telephone line, which the children inadvertently dialed, and then redialed almost 40 times in two weeks, said Boyle.

"Obscenity, profanity, pornography, you name it. I am outraged that my children could get hold of a number like that," said the Howard County mother.

Boyle's rude initiation into the world of "dial-a-porn," is symptomatic of a booming new industry, known as "audiotext," sweeping into dozens of major cities, including Baltimore, and starting next month in Washington, according to state and federal officials.

In October, the C&P Telephone Co. of Maryland became the first in the region to lease local phone lines to private companies offering recorded messages to the public. So far, the service has been available only in the Baltimore metropolitan area, where 23 companies have signed up, and 25 others will go into business next year, according to C&P spokesman Ralph Blunt.

One-third of the messages are pornographic or contain heavy sexual content, but others offer local horse race results, local event listings, horoscopes, consumer reports, sports trivia, rock music events, children's stories and jokes, Blunt said.

The Maryland Public Service Commission has received 200 complaints and inquiries about the service in the last three weeks, half of which have been aimed at the sexual content of the messages, according to Kenneth T. Hurwitz, the commission's executive director.

"When the commission originally approved C&P's request it was generally aware that sex-type services would be provided, but it was not aware of the degree of explicitness or the unsavory quality of the service," Hurwitz said.

Hurwitz said there is little the Maryland commission can do about dial-a-porn, but he said it is considering regulations to require a brief message informing anyone who calls this service that there is a charge and allowing the caller to hang up within five seconds without paying.

According to Mike T. Houghton, a spokesman for C&P in the District, similar audiotext messages will be offered to District and Maryland suburban residents starting next month. But the services will not extend to Virginia.

A C&P Telephone Co. of Virginia request for authority to lease local lines was withdrawn from the State Corporation Commission in October because of poor demand, said a company spokesman in Richmond. Because the company is not authorized to collect the service charge, suburban Virginia residents who attempt to call any of the message services in the District will get a "not in service" recording, the spokesman said. If they want to reach the numbers they would be forced to make a long-distance call.

Around the country, however, local telephone companies, newly independent and sometimes short on cash, are flocking to state regulatory agencies, seeking authority to lease local lines as a way to bolster revenues, according to officials. The calls are dialed like local numbers, but they cost 50 cents to $1 each. The charge appears on a customer's monthly bill, and the proceeds are split between the telephone company and the message service, according to telephone company officials.

Maryland's Hurwitz said C&P projected revenues in the audiotext industry of $9.8 million annually, of which $6.4 million is expected to be profit.

In the District, recorded message services are expected to produce up to $28 million a year by 1986, said Howard C. Davenport, the Public Service Commission's general counsel. The commission is expected to take that figure into account when it considers a pending C&P rate request for $79 million in new revenue, he said.

The audiotext industry literally began overnight, when the Federal Communications Commission told local telephone companies to discontinue recorded information services, effective Jan. 1, 1983.

This month, private message services were available in 40 cities, compared to eight cities in January, said Bob Thieringer, a market analyst with Bell Communications Research, which pioneered the computer technology that spawned the industry.

Everywhere service is introduced, dial-a-porn companies, usually based in New York City or Los Angeles, have been quick to invade.

Telephone company officials say they are powerless to control message content because of constitutional guarantees of freedom of speech. But that has not stopped thousands of people from protesting, said Sharon B. Kelley, an FCC lawyer.

"There has been a great public outcry from people and the Congress as well, because it's a very emotional issue involving children," she said. "We've received voluminous complaints; I mean thousands and thousands from around the country."

The controversy started in New York, where dial-a-porn numbers first became available as part of an audiotext system last year, according to officials.

"It was going through the schools like wildfire," said Martin Ashare, the county attorney of Suffolk County, a suburb of New York City. "At one point an administrator reported to me that he saw 20 kids at a pay phone. He picked up the phone and found out they were calling dial-a-porn. There were reports of parents with $200 and $300 phone bills."

Suffolk County filed suit to prohibit the messages, but withdrew it when the legal issues were eclipsed by actions in Congress and in the federal courts, he said.

Congress, which has been overwhelmed with complaints, made an effort to regulate pornographic messages last year. It passed a law prohibiting obscene or indecent communications for commerical purposes, except under circumstances provided for in rules that it ordered the FCC to draft.

The FCC considered measures such as access codes, some sort of blocking signal, and restrictions of the calls to credit cards, but it rejected them because they were either unfeasible or technically impossible. Instead, in June, the FCC issued rules limiting dial-a-porn service to the hours between 9 p.m. and 8 a.m. eastern time across the country, Kelley said.

But High Society Magazine, a leader in the dial-a-porn field, filed an appeal before the 2nd U.S. Circuit Court of Appeals in New York, shortly after the restrictions were issued.

In November, a three-judge panel, in a unanimous opinion, set aside the rules, declaring that the FCC had not conducted enough inquiry to prove that it had devised the least restrictive alternative to regulate the messages, Kelley said.

The judges also raised concerns about the constitutionality of the rules, but did not rule on that issue, she said.

Within the next three months, Kelley said, the FCC will publish notice in the Federal Register announcing another round of public comment on other possible restrictions. In the meantime, dial-a-porn companies are free to offer their services 24 hours a day.

High Society Magazine, an adult publication that features photos of nude women, leases three lines in Baltimore, several lines in other cities and probably will enter the Washington market, said publisher Gloria Leonard.

"I would certainly not deny they are erotic, sensual and provocative. However, the 57-second messages do not contain any of the hard-core language that George Carlin might use," she said.

Across the country, Leonard claims the High Society phone lines are called 500,000 times a day, a figure that is not disputed by telephone company officials. "Had we not been in court at virtually every level of the way practically this entire year, profits would have been great," she said.

While Leonard has no apologies for dial-a-porn, others in the audiotext industry are worried about their image.

"It's really a concern for us, because what happens on their line puts everything else in jeopardy," said Audrey Ingram, coowner of a Baltimore service that lists local events.