A 'phone in use' indicator, available in 2 versions. One which lights constantly when the phone is picked up, and one which flashes. Both circuits can possibly be approved by the (Danish) National Telecom Agency.
Each circuit draws about 20uA when onhook/3mA when offhook, and requires an offhook linevoltage of atleast 4V (4*Diode-Vf + LED-Vf) to work properly.
The 'flashing circuit' draws a nearly constant current, generating only an insignificant modulation (important, when having a highspeed modem connected on the same line).
The FET in each circuit must be a highvoltage type (atleast 250V), and the LED must be red (or yellow, but not green or blue, because of their higher Vf).
Using SMD components makes it possible to fit each circuit into a danish phone-line connector.

The standard circuit:

The flashing circuit: