DTMF (Dual Tone Multiple Frequency) is the oldest technique that created by AT&T to do signalling.

Source:

T101 Telecom 101 – 4th edition 2016
7″ x 9″ softcover book • 488 pages • 4th edition • published 2016
ISBN 9781894887038 (print)   ISBN 9781894887786 (eBook)