Trump has been handing out his cellphone number to counterparts around the world, urging them to call him directly to avoid the red tape of diplomatic communications. The practice has raised concern about the security and secrecy of the U.S. commander-in-chief's communications.
In today's world of cyber espionage, cellphone security experts say such a policy is not only unorthodox, but dangerous. Voice calls can be intercepted. A cellphone's signals to nearby phone towers can give up its precise location.
Even cellular networks are vulnerable. And knowing someone's number makes it easier to infect a phone with malware.
"Hillary Clinton's email server was like Fort Knox compared to Trump just carrying around a regular cellphone," said Andrew McLaughlin, former deputy chief technology officer for the Obama administration. "That's how bad the vulnerabilities are."
Running against Clinton for the presidency, Trump repeatedly criticized his rival for using a non-government email account while she was secretary of state. Trump argued that Clinton should not be given access to classified information because she would leave it vulnerable to foreign foes.