https://arstechnica.com/tech-policy/2019/11/impeachment-hearing-reveals-major-white-house-phone-security-fail/

In testimony yesterday before the House Intelligence Committee, diplomat William Taylor said that he had recently learned of a phone call between George Sondland—the US ambassador to the European Union—and President Donald Trump. Taylor, the senior diplomat for the US in Ukraine, said that his staff overheard Trump during a call with Sondland while at dinner with the ambassador at a restaurant in Kiev.

The contents of that discussion—that Trump asked Sondland about "the investigations" Trump wanted Ukraine to conduct as an alleged condition of releasing military aid—may or may not be damaging to the president's case that he was not seeking foreign assistance for his 2020 presidential campaign. But as anyone in national or diplomatic security will attest to, an open phone call between the president and an ambassador regarding topics of diplomatic interest in a public place like a restaurant—a place where any foreign intelligence organization could be monitoring for collection purposes—would be a major breach of operational (and national) security.