Public service company offerings don't get much sleazier than dial-a-porn telephone messages.

Telephonic sex-talk has mushroomed into a $2.4 billion-a-year business. A large chunk of the money goes into telephone company coffers and the rest to those who sponsor the messages.This contamination of telephone lines has led to tens of thousands of complaints. Most are directed at the availability of the material to children. Congress and the Federal Communication Commission have tried, with mixed success, to restrict access to the messages to adults.

After dial-a-porn services began sprouting in 1983, Congress made it a crime to make indecent or obscene calls available to minors or unwilling adults. The FCC has tried several ways to enforce the law but has run into legal problems involving the First Amendment's guarantee of free speech.

Most telephone companies say they don't like providing the service, even as they rake in the profits. Some telephone companies have offered programs under which customers can have their lines rendered incapable of accessing the pornographic message centers, but this usually requires the customer to pay a fee for the blockage.

A better plan would be an across-the-board blockage except to customers who want dial-a-porn service and are willing to pay an extra fee for access to it. This, of course, probably would lower the profitability to telephone companies.

At least one civic-minded phone company, Southern Bell, has refused to let dial-a-porn entrepreneurs use its lines, and a federal court of appeals has upheld the action on the ground that the company doesn't have a First Amendment duty to carry such messages.

Sen. Jesse Helms, R-N.C., says a complete federal ban on dial-a-porn telephone messages is the only sure way "to end the horrendous practice engaged in by people with warped minds and telephone companies who cooperate."

Legislation to do that is close to approval by Congress, but there's no assurance that it would survive a court challenge based on free speech rights.

In the meanwhile, telephone companies ought to be as hard-nosed as the courts will allow in protecting their lines and their customers, especially youngsters, from dial-a-porn filth. Public service companies have an obligation to do more than just look to their profits.