From Wikipedia: Invented by engineer August Kruckow, the dial tone was first used in 1908 in Hildesheim, Germany.[1] In the United States, the dial tone was introduced in the 1940s, and became widespread in the 1950s. read more
A dial tone is a telephony signal sent by a telephone exchange or private branch exchange (PBX) to a terminating device, such as a telephone, when an off-hook condition is detected. It indicates that the exchange is working and is ready to initiate a telephone call. read more