When this device is "on hook", the speech circuit is disabled & the ringing circuit is connected
The base of the transistor is given a xed bias by a voltagedivider 
connected... [DOC] [TLE] Patent US2823267 - Delay circuit for amplifying device - Google ... [PAR] Another object of the invention is to disable a circuit including an amplifying ... by 
`a breakdown diode connected in series with the signalling device.