Phlox was a member of the Hierarchy. While engaging in a reconnaissance mission, Phlox detected the USS Voyager, but instead of interfacing with its sensors, he interfaced with The Doctor instead. At the time, The Doctor was experiencing daydreams, which made Phlox believe that the exaggerated things The Doctor was thinking of were real.
When data of the events were transferred to the Hierarchy, an attack was planned. When Phlox realized that what he saw were daydreams, he was afraid to tell his superiors because they didn't tolerate mistakes and he was afraid he would be punished. Phlox felt that he had gotten to know The Doctor, and he warned Voyager of the alien attack. Due to this action, The Doctor was able to stop the attack and prevent Phlox from being punished for his mistakes. (VOY: "Tinker Tenor Doctor Spy")