... microphone.*Low noise, Low echo, clear speech, low price.*Built in digital audio disposal circuit , two step dynamic de-noise disposal ... design, eliminating echo in big hall.*0.8mm pinhole design, clear picking voice.*Pinhole style, little volume, suitable for jail room and other ... required: 3 x 0.5mm2 RVVP cable, 4 core phone line, network twist line, etc.*Built-in over-loading protection and ...