The field transceiver is used to create phantom or superphantom lines on field two-wire lines. The phantom line allows a third line (pair) to be built after two lines (pairs) without interfering with each other. The first two lines must be built from two-wire lines. The phantom cannot be used for UB or AUT telephones, only for MB system field telephones (TP 25, etc.).
Source: Spoj-51-2 (1962)
Source: Spoj-51-2 (1962)