You sent me down a rabbit hole of wanting to know more about this phone!
Turns out it is the CB version of another common Soviet phone. So-called “CB” phones (short for Central Battery) are single-line, self-contained, point-to-point systems used in special situations, such as between a security office and the main gate of a compound, that sort of thing. They are the high tech version of two cans on a string.
#TIL!
(For the record, I like Rob’s explanation better. 
)
