gives you a visual indication when a line is in operation. The two AA battery cells should provide enough power for about one year of operation.
http://www.imagineeringezine.com/PDF-FILES/4phostat.pdf
I have used this type of circuits for quick testing of telephone equipment. This is a quick hack that does not exactly resemble the characteristics of the real telephone network, but is enough for this kind if applications.
http://www.tkk.fi/Misc/Electronics/circuits/telephone_test.html
Acompact, inexpensive and low component count telecom head- set can be constructed using two readily available transistors and a few other electronic components. This circuit is very useful for hands-free operation of EPABX and pager communication
http://www.electronic-circuits-diagrams.com/telephonesimages/telephonesckt13.shtml
It only monitors one phone line.
http://www.imagineeringezine.com/PDF-FILES/1phostat.pdf