Touch Tone fourth-column transaction application [telecom]

In the 1970s the Bell System developed a Transaction Telephone and support network to provide for things like checking account balance and credit card verification. It developed a series of telephone sets, networks, message switching, software, and testing administration facilities to meet this service.

A series of article describing the service may be found at:

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In one article, they make use of the fourth column (a-d) of Touch Tone buttons not found on normal telephones (but often mentioned for Autovon service). These keys are used to send special signals.

The specific article may be found at:

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The telephone sets include a keypad. However, this keypad uses a different type of switch than found on Western Electric Touch Tone pads; rather used on calculator keypads.

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