BL27 Field telephone or “Phase Phone” .

A cheaper more Robust Alternative to the SOUND POWER Phone.

Can be used   in many commercial/industrial locations such as: airports, fire and police rescue crews, public utilities,schools, vaults, refrigeration plants, civil defense, bridge installations,
ski slopes, oil fields, parks and forest, railroads, salvage yards, sporting arenas, shipyards, diving projects and geophysical operations where power is not available, to talk to a person up to 10km away on two wires.

Widely  used for phase testing on Electricity supply lines where it is referred to as a phase phone.Lightweight and portable, the equipment is comfortably used for in-plant and
outside maintenance, construction and repair; electrical contract
installations, public utilities, radio, television, telephone installations
and shipboard operation.

Uses a 9 Volt battery (Alkaline) with a rock switch one way to call and one to talk so it is off in the normal position. It has a Led which comes on when battery is low.
Many application  where two way radio will not work due to
obstructs such as underground in tunnels etc.. It is also cheaper than a radio and battery can last a long time as the talk draws very little power