A Phone Box is a stationary object found inside the Hunting Lodge in Episode 2: Luminance Fugue in Wintermute. It a has no functionality when first found, but while exploring the Lodge during the Aurora, the phone will begin ringing. Approaching and opening the cover will cause the phone to stop, leading to no further interaction.
Trivia[]
The phone may be on a party line, and could be foreshadowing Molly and Astrid Greenwood speaking to one another in Episode 3: Crossroads Elegy. It could also be Jeremiah speaking to someone else, though this is unlikely considering the lack of functioning electronics in his cabin.