If only one voice connection is required, old field telephones can be used sensibly. Under favorable conditions (good cable, no interference, optimal cable routing), the range for such a field telephone is up to 15 km in civil engineering (laying on the ground) and up to 40 km in building construction (laying over masts, trees, etc.) !
The range of used technology is large. For a secure connection, however, you should not fall back on old World War II telephones. "Newer" phones from the 1960s onwards are better.
If long overhead lines are laid, overcurrent or Surge arresters should be considered as protection for the connected telephones.