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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Lisa or Jeff
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