The woman who ran the outcall massage service section of what the FBI has termed the Washington area's "largest and most sophisticated commercialized prostitution business" pleaded guilty yesterday to interstate prosecutors in the final stages of their 18-month-long investigation.

Phyllis Darlene Paige became the third major figure in the operation to plead guilty to racketeering charges in as many weeks. The operation, which officials say was run by Louis Michael Parrish, reportedly grossed $1 million annually.

Two weeks ago federal officials obtained a guilty plea from Chyde Ballard (Skip) Stovall, a top Parrish lieutenant with comprehensive knowledge of Parrish's 11 massage parlors and outcall massage services.

Last week they obtained a guilty plea from Gray Marion Van Ryzin, a trusted Parrish aide with knowledge of the massage parlor operations themselves.

Yesterday Paige, 29, pleaded guilty to one count of interstate racketeering in connection with her lengthy employment at the Dial Us Outcall Massage service, located in Alexandra. U.S. Attorney William B. Cummings told U.S. District Court Judge Albert V. Byran Jr. that on Feb. 27, 1977, Paige sent a woman from Alexandria to the Washington Hilton Hotel in the District of Columbia for purposes of prostitution.

Paige, a heavy-set woman with long brown hair, told Bryan that the charge was true and that she had agreed to cooperate fully with prosecutors. Bryan released her on a $1,000 personal recognizance bond and set sentencing for Nov. 3. Paige could be sentenced to up five years in prison and a $10,000 fine.

The outcall massage parlor service run by Parrish advertised for customers in telephone directories and hotel brochures and distributed leaflets and business cards at major hotel and convention centers, according to officials. When a customer called one of several dozen telephone numbers used in the operation, an operator took the customer's name and telephone number, and called him back to verify the name and address, before dispatching a masseuse, according to an FBI affidavit filed last year in connection with the investigation.