Call waiting
History
Call waiting was introduced to North America in the early 1970s when the first generation of electronic switch machines built by Western Electric, Electronic Signaling System 1 started to replace older mechanical equipment in the old Bell System local telephone companies. At first, some smaller municipalities were able to offer customers call waiting only on a specific phone exchange (e.g., phone customers in Trenton, Michigan initially had to have a phone number starting with 671 to have call waiting, since 671 was at that time the only exchange in that area served by one of the new ESS switches), but as demand for it became more widespread, it eventually became available on all phone exchanges as the older equipment was phased out.
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