A method and apparatus that generate a 2100 Hz answer tone, similar to the answer tones sent by facsimile machines and modems, upon answering an incoming call. The anti-noise signal causes cancellation of ambient audio sounds that appear at the reference microphone. As a result, of a pair of the transistors connected in series, namely either the transistors 11 and 14 or the transistors 12 and 15, when one transistor turns ON, the other transistor turns OFF. The chamfer is formed in such a way that the angle enclosed by the large surface side edge, is larger than an angle enclosed by a narrow side edge. By this method, sintered articles of polybenzimidazole resins having no weak parts can be obtained in high yields.