This is off the top of my head, as you or someone else can look up the details. A typical single line RJ11 phone jack only uses two wires. The two wires it uses are the center two wires, typically red and green.

These two wires sit somewhere at like 9-24volts which powers the phone, when the phone rings, these jump to somewhere in the 80volt range. The phone signal is just an analog audio signal sent over the two wires just as you would send audio to a low power speaker.

A neat trick is that you can connect two phones together by simply connecting one side of their wires together and the other side through a 9 volt battery. You can then talk to someone on the other phone.

Some good resources for information and wiring diagrams.

http://en.wikipedia.org/wiki/RJ11,_RJ14,_RJ25

http://www.tech-faq.com/telephone-wiring.shtml

http://cableorganizer.com/articles/network-instructions.htm

http://www.tech-faq.com/telephone-voltage.shtml

etc. More found using a simple search of the internet.

What's the application as only approved equipment (and some approved electronics) are allowed to be legally connected to the telephone network (for obvious reasons)?

The 2nd to the last link was very useful thanks. As far as 80 something volt ring signal I will use an optoisolator that can handle that voltage.

the RJ11 ( with tab down ) counts from left to right...1,2,3,4 and only uses pins 2 & 3 if it is an analog 2 wire phone. Pin #2 is tip (+) and pin #3 is ring (-). In normal on-hook conditions, there is less than 6 volts DC on the pins, and off-hook you will draw dial tone from the local central office. When there is a call coming in you will see a negative -48VDC, the old timers call this ring down current. HTH