Presidents generally place calls on one of several secure phone lines, including those in the White House Situation Room, the Oval Office or the presidential limousine. Even if Trump uses his government-issued cellphone, his calls are vulnerable to eavesdropping, particularly from foreign governments, national security experts say.

“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.

A president “doesn’t carry with him a secure phone,” Chollet said. “If someone is trying to spy on you, then everything you’re saying, you have to presume that others are listening to it.”

The caution is warranted even when dealing with allies. As German Chancellor Angela Merkel’s learned in 2013, when a dump of American secrets leaked by Edward Snowden revealed the U.S. was monitoring her cellphone, good relations don’t prevent some spycraft between friends.