From the show
Get Smart - "Another of the show's recurring gags was the Cone of Silence. Smart would pedantically insist on following CONTROL's security protocols; when in the Chief's office he would insist on speaking under the Cone of Silence--two transparent plastic hemispheres which were electrically lowered on top of Smart and the Chief--which invariably malfunctioned, requiring the characters to shout loudly to even have a chance of being understood by each other, and even then, most of the time that failed. In one episode, the device worked so badly that an outside observer, who could hear everything spoken inside, had to relay messages to the people inside because they could not even hear each other. At other times, the Cone of Silence would malfunction while being lowered and fail to stop at the proper desk level; the device would then repeatedly crush down onto Agent 86 and the Chief, resulting in loud anguished screams. The Cone actually worked as intended once. However, at the end of the conversation, the Cone malfunctioned leaving the Chief trapped within, with silent screams of frustration as Agent 86 walked away."
Thus the "WHAT!" I always remember the Chief and Smart having a hard time hearing each other while using the Cone of Silence.