President Donald Trump has been handing out his cellphone number to world leaders and urging them to call him directly, an unusual invitation that breaks diplomatic protocol and is raising concerns about the security and secrecy of the U.S. commander in chief’s communications.

Trump has urged leaders of Canada and Mexico to reach him on his cellphone, according to former and current U.S. officials with direct knowledge of the practice. Of the two, only Canadian Prime Minister Justin Trudeau has taken advantage of the offer so far, the officials said. . . .

“If you are speaking on an open line, then it’s an open line, meaning those who have the ability to monitor those conversations are doing so,” said Derek Chollet, a former Pentagon adviser and National Security Council official now at the German Marshall Fund of the United States.