Circuits have been published on earlier occasions that keep an eye on the telephone line. This simple circuit does it with very few components and is completely passive. The operating principle is simplicity itself. The circuit is connected in series with one of the two signal lines. It does not matter which one of these two is used. When the telephone receiver is lifted off the hook, or the modem makes a connection, a voltage will appear across the four diodes. This voltage is used to drive the duoLED.
Depending on the direction of the current,either the red or the green part of the duoLED will light up. In some countries, the polarity of the telephone line voltage is reversed after a few seconds. This does not matter with this circuit since a duoLED has been used. Depending on the polarity of the line, the current will flow through either one branch or the other. The 22-Ω resistor is used as a current limiter, so that both colours are about the same brightness. The duoLED can be ordered from, among others, Conrad Electronics (part number 183652). You can, of course, also use another, similar LED. For the diodes use the ubiquitous 1N4148.