Sos intercom KNZD-11 is often called emergency intercom, which is mainly used in public places. When an emergency occurs, tourists can use intercom for help. This intercom is a terminal device that transmits voice in both directions via voip. It is mainly composed of call button, indicator light, indicating sos. Intercom can only make point-to-point calls. How SOS intercom works Intercom communication is a communication technology that uses sound and electric energy to convert each other, and uses "electricity" as a medium to transmit language. The simplest form for two users to communicate is to connect two sos intercom with a pair of lines. When the speaker is speaking into the microphone, the vibration of the vocal cords stimulates the air to vibrate, forming a sound wave. Sound waves act on the microphone and cause it to generate an electric current, which is called voice current. The voice current is transmitted along the line to the receiver of the counterpart radio n. The receiver acts just the opposite of a microphone-it converts electrical current into sound waves and passes them through the air to the human ear. This completes the simplest call process Full keypad with memory, 10 button programmable telephone. Weather resistant to IP55 standard, service temperature range from -30 degree to +75 degree. Lightening protection to ITU-T recommendation K.21. Heavy Duty handset with hearing Aid compatible receiver, Noise canceling microphone. Handset with industrial strong armored cord . Metal keypad keep waterproof and dusty proof.