This acoustic telephone call signalling
device is particularly designed for use in areas exposed to extreme environmental conditions. The tough housing withstands rain,
high humidity, heat and cold as well as the influence of acids, alkalis, and petro-chemical products. The user can
select between a single-tone, two-tone, three-tone or warble ring with the help of internal dip switches. Four
different tone sequence frequencies can also be selected in a range between 5 and 20 Hz.