Omnicom Media's India chief was frustrated. It was Oct. 5, 2023, and a rival was trying to poach the U.S. firm's client by offering lower prices, just weeks after global advertising agencies and broadcasters struck secret pacts on ad rates in the South Asian country.

The attempt to woo the client violated the agencies' agreement, Omnicom Media's India CEO Kartik Sharma wrote in a WhatsApp group comprising a who's who of advertising, according to excerpts of the discussion documented by antitrust investigators.

"This kind of practice is not in the spirit of what we are collectively trying to achieve," Sharma wrote, without identifying the parties.