Telephones are now readily and cheaply available: two identical ones and a handful of components enable a simple house telephone to be set up.
Since the two telephones are connected in series as shown in the diagram, half the supply voltage exists across either of them. Neither buzzer will sound since the potential across zener diodes D1 and D3 is below their breakdown voltage.
If, say, the handset of telephone 2 is lifted, a virtual short circuit ensues across this telephone. The potential across telephone 1 then rises to almost the supply voltage. Breakdown then occurs in D1, which causes a sharp increase in the reverse current through the diode. The buzzer will then sound and the LED light. If then the handset of a telephone is lifted, the supply voltage is again divided symmetrically across the two telephones, which is sufficient for carrying out a conversation. The buzzers may be continuous-tone or intermittent-tone types to personal preference. Similarly, the LEDs may be standard or flashing types.
The power supply may be a standard 12-V mains adapter. When the supply voltage is too high. the buzzers will sound even when both handsets are lifted. If dissimilar telephones are used, one or both zener diodes need to be replaced by different types to ensure that during quiescent operation the voltage drops across the telephone are identical.