Phillips Petroleum Co. stock continued to trade heavily on the New York Stock Exchange yesterday as rumors swirled that several investors were amassing stakes in the company as a prelude to a possible takeover offer.

Phillips was the day's most active issue, up 75 cents to $48.75 on volume of nearly 2.9 million shares. Phillips has been consistently very active in the past two weeks, and has risen $4.50 in that time. "You've got to believe that something is going on with that kind of trading volume," said Rosario Ilacqua, an oil-industry analyst at L. F. Rothschild in New York. "I can't believe that it's just brokerage-house speculators."

Two developments during the day heightened interest in the stock. Minneapolis financier Irwin L. Jacobs confirmed he was a "heavy player" in Phillips stock -- although he refused to comment on one Wall Street rumor that he was assembling a 5-million-share block of the company's stock -- and Phillips set Feb. 1 as the record date of ownership for shareholders who wish to vote on its proposed recapitalization plan. The date of the actual vote has not yet been announced.

Analysts have identified investor dissatisfaction with the recapitalization proposal as the reason for the high level of interest in Phillips.

The biggest opposition to the plan is said to come from arbitragers, professional stock speculators who have been the biggest buyers of Phillips recently. Analysts believe the arbitragers are accumulating stock this week to be eligible to vote on the plan. It is believed that they then would try to vote it down and seek an alternative proposal, either from the company or from a third party.

Wall Street speculation is that the third party could be Pennzoil Co. or another big oil company.

An offer also could come from a group of investors including Jacobs and other big-time speculators such as Carl Icahn or Ivan Boesky, perhaps in league with Pennzoil, analysts said.

Phillips yesterday declined comment.