At the start of what promises to be a record season for mail-order holiday shopping, the Federal Trade Commission warned yesterday that consumers forfeit their mail order rights when they place their orders by telephone.

Under the agency's Mail Order Merchandise Rule, the consumer who buys by mail normally is entitled to delivery within 30 days or the time specified in the company's advertising. If the company can't meet that deadline, it must notify consumers so they can either ask for a refund or accept the delay.

The consumer loses that protection when he makes a phone order and bills items to a credit card account, according to FTC consumer protection specialist William S. Sanger.

"So we recommend you mail in your payment," Sanger said.

Sanger's warning came during a press breakfast hosted by FTC Chairman James C. Miller III, who has been advocating that the mail order rule be expanded to cover phone orders. Miller said the agency is developing information on which a phone order rights' rule could be based, if the FTC determines such a rule is needed.

Agency officials said that mail-order companies typically don't distinguish between mail orders and phone orders. However, if a consumer were to have a problem, he would have more protection if he had placed the order through the mail rather than on the telephone.

The Miller briefing is part of the agency's national campaign to provide consumers with tips on how to avoid problems this holiday season. FTC officials are appearing on radio and television shows to talk about the tips and distributing free leaflets that cover shopping by phone or mail, credit billing problems, credit and charge-card fraud and warranties.

Leaflets can be ordered from the FTC by calling 523-3598 or writing the agency at Sixth Street and Pennsylvania Avenue NW, Washington D.C. 20580. Consumers should specify the topic they want.

In discussing holiday shopping problems, Sanger said that mail-order complaints are the number one problem for consumers. "We anticipate about 9,000 complaints this year, a slight increase over last year's 8,800," he said.

Sanger said the mail-order business is expecting a record year in 1984, with sales expected to reach $44 billion, compared to last year's $38 billion.

About half of those transactions occur during the holiday season, he said.

The FTC offers these tips for mail-order purchases:

* Determine the shipping date and order early enough to allow plenty of time for delivery.

* Examine the company's return policy. If the advertisement or the catalogue doesn't tell you what that policy is, telephone before ordering and inquire.

* If you aren't familiar with the company or are uncertain about its reputation, check with your local consumer protection agency or the Better Business Bureau to find out if there are complaints against the firm.

* Keep a record of the company's name and address, the date of the order, the ad or catalogue from which you ordered, the order form and canceled checks or charge record.

To resolve a problem, consumers should contact the company first, Sanger said. If that doesn't work, he said, they should contact the consumer office in their area and the Direct Marketing Association, an industry group at 6 East 43rd St., New York, N.Y. 10017.

The FTC does not attempt to resolve individual complaints, but may get involved when there is a pattern of violations. Consumers should send a copy of complaints to the FTC.