For greater mobility, various radio systems were developed for transmission between mobile customer stations on ships and automobiles from the 1930s by the mid-1900s. This gave rise to landline telephone service in which each telephone is connected by a pair of dedicated wires to a local central office switching system, which developed into fully automated systems starting in the early 1900s. Being impractical beyond just a few customers, these systems were quickly replaced by manually operated centrally located switchboards.

The first telephones were directly connected to each other from one customer's office or residence to another customer's location.

A telephone converts sound, typically and most efficiently the human voice, into electronic signals suitable for transmission via cables or other transmission media over long distances, and replays such signals simultaneously in audible form to its user. A telephone, or phone, is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be heard directly.

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