Additional Information:

When connecting the phone line to the circuit, just connect without bothering where to connect the 'Red wire' and where to connect the 'Green wire'.

The voltage and current values measured in your phone line may vary according to the specifications of different service providers in different countries. In that case, you may sometimes face with a busy line problem (engaged phone line). But it can be easily overcome. Here is the theory. The higher the resistance of R1, and the lower the resistance of R2 the brighter is the light. But on the other hand it cause more tendency for busy line. The R1, R2 values given here are the best combination worked for my phone line. It has a voltage of 48 v and a current of 22 mA.

So if your line gets busy after connecting this, first try to replace the resistor R2 with a some what larger value (say 270, 330 ohm). This should solve the problem. If not, try to reduce R1 (say 3.3k, 1k ). With your dedication you can find the best combination for your phone line. If you are too lazy to experiment with this, give your fist try with R1=270 ohm and R2 = 1 k ohm. This will work for sure, but you will loose a little amount of brightness of the light.

The number of LED's to be connected in series with this is not strictly 12. You can even use a single LED.

If you connect a lesser the number of LED's with this in series (say 4, 5), your light won't switch off even when you pick up the receiver. Fine, It's OK and it's better. But if you try to increase the number of LED's beyond 12 you will notice that the brightness of each bulb fades out gradually. Anyway, upon your interest you can have a go, and see whether you will get fully bright 15 LED's with your phone line. All the best for your try.