It is generally considered that current rather than voltage is what kills. Of course the two are related but it is possible to get very high currents from low voltage sources if the resistance is low.
Ordinarilly, point skin resistence is high enough to make low voltages inconsequential. But where skim contact area is large, such as if hands were wet with some conducting liquid, lethal currents can definitely be generated.
I wouldn't doubt at all that field telephones could electrocute under the right circumstances.
[...electrocuted by 12 volts...]
How did they carry the phones into the water (in the jungle)?