One of Wall Street's ugliest spats ended last week with a phone call.

Early last year, Wall Street was captivated as Carl Icahn and William Ackman, two of the nation's top "activist" investors, traded insults on national television for 30 minutes. Mr. Icahn called Mr. Ackman a "crybaby in the schoolyard." Mr. Ackman retorted that Mr. Icahn was "not an honest man."

The hatchet was buried on Thursday. Mr. Ackman spoke with Mr. Icahn's assistant on April 24 saying, "I am calling to forgive Carl," according to Mr. Ackman. Mr. Icahn, 78 years old, returned the call and told Mr. Ackman, 47, that "it is a blessing to forgive," Mr. Icahn said, "and I forgive you."

The development suggests that the duo, who famously clashed on their views about nutrition company Herbalife Ltd., could team up on activist investments, where investors push for broad changes at companies or try to move prices with their arguments. Neither would name a target or timetable.

In the future, Mr. Ackman said: "There is a much greater possibility that we are on the same side than the opposite."