Oleg Cassini says he doesn't really like to talk on the phone. But that hasn't stopped him from getting into the designer telephone business.

Telephones are the newest way for fashion designers to ring in big bucks. With the change in Federal Trade Commission regulations and the breaking up of Ma Bell, the telephone has become a full-fledged over-the-counter consumer product ripe for designer treatment, just like jeans and perfumes.

"Sometimes the look of a marvelous apartment can be ruined by the telephone," said Oscar de la Renta, adding that comfort as well as color are the focus of his changes.

"There is no reason why a telephone shouldn't be attractive without being cutesy," said Bill Blass, who has designer phones in stainless steel and chrome.

Perry Ellis likes to watch his friends talk on the phone, particularly when there is a mirror nearby. "I can't tell you exactly what they do but the combination of the phone and the mirror makes them do wonderful things," said Ellis. It's no surprise that several months ago, when Ellis was asked to design a new telephone, he created one within reach of a mirror.

Geoffrey Beene and Hubert de Givenchy also offer their own phone lines.

As for Cassini, his name will be carved in expensive woods or painted on porcelain phones, though he said during a brief conversation by phone from Milan, "Some people have telephonitis, others are afraid of the phone. I really don't feel comfortable on the phone. I'd really rather talk to someone in person."

Speaking on a panel at the Consumer Electronics show in Las Vegas earlier this month, where more than 200 exhibitors displayed a huge variety of phones, Alfred Franks, a vice president of American Bell Inc., predicted that 30 million phones would be leased or sold in 1983, and 10 million of them would be new units. Some of American Bell's phones look like Mickey Mouse, Pac-Man, cigar boxes, candlesticks, bagels and even antique phones. None are designer phones, nor will they be anytime soon. "We looked into it and decided it was not for us," said Michael Tarpey of American Bell Consumer Products. "It is a function of price. We would have to increase the price to pay the fee, and decided that is not for us," said Tarpey, who estimated that the increase for designer input would have been 10 to 20 percent.

But there is another side to the story. "Women of America have come to believe designer clothes and products have quality and style," said Duffy Fankboner of Telephone Marketing Associates, licensing agents for designer telephones. He sees a potential market of 140 million residential telephones. "If designers get 2 to 5 percent of that market, that's not bad," said Fankboner, who has a tie line to Blass, Ellis, Givenchy and De la Renta. "It could be one of the most lucrative licensing arrangements," said Fankboner, who would only say that "up front" money plus royalties might make a million dollars for a designer. Fankboner expects to have his designer lines ready for next Christmas, with price tags starting at $200.

"Telephones are as personal as colognes or clothes and something we use every day. Why shouldn't they be attractively designed by designers?" said Larry Kifer, chairman of Technicom International, the company Beene designs for. Kifer expects Beene phones to retail from $59.95 to $199.

Even before American designers got busy with phone lines, two of the most aggressive European designers already were selling designer phones. Pierre Cardin introduced a line of a dozen different phones, all emblazoned with his signature, more than a year ago. And for almost as long, Gucci has been selling a flat, sleek, silver telephone with a silver signature stirrup on the receiver. The price tag: $1,500. The New York store sold out of the 12 they had in stock, although the made-in-Italy phones "had many problems working here," according to a company spokeswoman. Corrected versions are due in Gucci shops soon, she said.

Just what designers can do to a phone is limited by more than the imagination. The Federal Communications Commission has a certification program for telephone equipment to insure that, whatever its appearance, a telephone has the inner workings to function like, well, a phone.

While some new phones may incorporate sound and security systems and new-fangled dials, the designers' concern is totally esthetic. "I tried to take the squareness away from the phone, to make the edges more round and the phone more sensual," said Beene. He approached the phone design in the same way that he worked on his perfume bottle, "as a sculpture that one handles," he said.

It hadn't occurred to Beene to design a phone until he was approached six months ago by Technicom International, Inc., designer and manufacturer of business and residential telephones. "It is the first environmental thing I've done apart from sheets," said Beene, who wants to create more things for the home. Shortly he will introduce upholstery fabrics. "Why shouldn't a telephone be on the same taste level as other things in the house?"