The technology of cancelling button press tone signals.
People on the other end of the line have virtually stopped using pen and paper. They now use their telephones to record the necessary information.
While doing that, they produce tone signals that may be annoying for the operator.
One may well imagine how hazardously loud these tones are, if they may even be heard by people, standing close to the operator.
Real time cancellation of tone signals is a demanding problem.
Classic noise cancellation systems are of no use in this case, but we know how to solve the problem.