Phillips Petroleum will cut its worldwide work force by 7% to 10% in an effort to trim $200 million in annual operating costs. "These changes are essential in order to improve profitability in a very tough environment," said C. J. Silas, chairman and chief executive. The Bartlesville, Okla., company currently employs about 22,500 people worldwide, about 2,500 fewer than in late 1985, when Texas oilman T. Boone Pickens Jr. attempted to take it over, a spokesman said. The company fought another takeover attempt in early 1986 backed by New York financier Carl C. Icahn. The takeover efforts left Phillips deeply in debt and forced it to sell several of its subsidiaries. Silas said the company would consolidate its exploration and production unit with its gas and gas liquids unit. Phillips had net earnings in 1986 of $228 million on sales of $10 billion.