If you feel
that somebody is tampering with your telephone line you might find this little circuit
useful. It detects if there is another telephone connected to the line, if there is a
short or an open line. Sound and a flashing light will tell you which is the current
situation. The speaker is practically cut out during a normal conversation thus preserving
privacy, only the LED will flash occasionally. The circuit does not require any battery
and takes the supply from the telephone line itself. The transistors used are wired in a
reversed biased fashion thus behaving as oscillators. You might try the 2N2222A as an
alternative (not tested). This monitor is, of course, suitable only for analogue lines.
Watch the polarity of the input line: the circuit will not be damaged by a polarity
reversal but it will not operate correctly.