The telephone phantom box was used to attach additional telephones to existing telephone lines. It was used at field telephone exchanges. The principle involved tapping into the secondary winding of transformers inside the box. If the resistance of the wires was the same on both sides of the winding, an excellent voice reproduction could be heard. This particular unit was made in 1941 by Wilhelm Sedlbauer of München.

 

 

The Phantom Box

 

 

The schematic showing serially connected 21 Ohm resistors. The manufacturer's mark can also be seen here.

 

 

Wire nuts on top of the box.

 

 

A better photo of the top.

An alternate plug-based connector for the telephones.