According to New York Times, discovery was made by White House officials who were alerted by individuals working within these foreign governments.
According to the New York Times report:
In what amounts to a marriage of lobbying and espionage, the Chinese have pieced together a list of the people with whom Mr. Trump regularly speaks in hopes of using them to influence the president, the officials said.
Among those on the list are Stephen A. Schwarzman, the Blackstone Group chief executive who has endowed a master’s program at Tsinghua University in Beijing, and Steve Wynn, the former Las Vegas casino magnate who used to own a lucrative property in Macau.
The Chinese have identified friends of both men and others among the president’s regulars, and are now relying on Chinese businessmen and others with ties to Beijing to feed arguments to the friends of the Trump friends.
The names of the White House officials and individuals working within these foreign governments have remained anonymous for security reasons.
They said the Chinese have determined a list of contacts with whom the president speaks most frequently.
The president keeps two official iPhones with modifications by the National Security Agency, and a third personal phone for personal contacts, White House officials said.
But using any cell phone poses some risk. The report indicated Trump prefers using his cell phones over the White House’s more secure landline.