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Dial tone is a feature common to almost all telephone systems, and is taken for granted on a daily basis by billions of people all over the world. It is used to audibly indicate a live, functional phone line, signaling a ready state for dialing digits to originate a call. Before the introduction of dial tone, direct dial telephone subscribers just picked up the receiver and started dialing, unsure that the equipment was ready to receive calls. If the call didn't go through, they just hung up and started over. Of course, most people at that time didn't have direct dial subscriptions, and required a live human operator to manually connect their telephone calls.

Dial tone was a technological by-product of automatic switching equipment, which was invented in the United States by Almon B. Strowger's Automatic Electric Company, and first put into commercial service in La Porte, Indiana in 1892. Direct dial switching was initially controlled by a series of primitive push buttons, not by a dial. A large dial mechanism was first used in Augusta, Georgia in 1896, and required a two wire line plus a ground connection. A smaller dial was introduced in 1907, and a two wire version without the ground (similar to that still found in use today) was put into limited service in Pontiac, Michigan in 1908, with the first major installation in Lansing, Michigan that same year.

Engineers at Automatic Electric (AE) were the true inventors of dial tone, and this feature was introduced commercially in Germany in 1913, where AE supplied that country's first dial exchanges. The Germans were the first to recognize the benefits of dial tone, and soon Siemens and other suppliers were incorporating this feature into their dial equipment under license from AE. As a result, the distinct sound of dial tone recognized today differs from country to country, as its implementation by a variety of equipment suppliers became more widespread.

Ma Bell takes credit for the introduction of dial tone in the United States with the installation of the first automatic exchange in America, which occurred in Norfolk, Virginia in 1919. The debut of the Bell System exchange was achieved through technologies developed by their supplier, Automatic Electric. Previous AE installations in the US did not feature dial tone, and were for independent telephone companies. Strowger's dial tone system was reluctantly regarded by the Bell System as "commercially acceptable", and for many years Automatic Electric supplied dial switching equipment to the Bell System, while Western Electric made Strowger-type equipment under license from AE.

When Lucent Technologies was created in the 1990s (as a spin-off of AT&T Technologies), Lucent came out with a series of ads in the US which reviewed the many technological achievements of Bell Labs. Several ads featured the claim that Bell Labs was the inventor of dial tone, but this is merely wishful thinking on their part.

Reference:
Hershey, Harry E. Automatic Telephone Practice. 1954

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