The use of The PKI 2755 Duplex Communication incl. Wireless earphone communication devices is especially valuable when top secret observation is required. The pocket device works on full-duplex-procedure, i.e. transmitter and receiver use different frequencies at the same time which allows a communication like on the phone. Listening and speaking can be done simultaneously without any switching. In combination with our wireless earphone PKI 2440 a totally concealed communication is possible. A coil connected to PKI 2755 transfers inductively the signals in a range of up to 20cm to the earphone. The coil has to be fixed on the shoulder, underneath the shirt. Then it is perfectly disguised and no cable connection to the earphone is required. The earphone is powered by an integrated coin cell battery. PKI 2440 is so small and fits in every acoustic meatus, a perfect equipment and undetectable.