Schematics / Telephone Circuits
Phone Line Monitor
Here's a very helpful circuit idea from EDN which will aid in the design of any equipment that interfaces to the phone line...
Telephone Interface Circuits
First of all, I must officially advise against connecting anything other to the telephone line than equipment approved for the purpose by the telephone company or some other regulatery body. Telephone company tend to be very strict about unauthorized gear hanging on their lines, and if something does go wrong with your gadget (like putting dangerous voltages to telephone line) you will be in deep trouble....
Line Status Indicator
When only one phone line is available for two phones, each time you want to make a call, someone may be using the second phone. A simple circuit lights an LED, which indicates whether the line is free...
Telephone Transmitter 
We recently received a request for an electronic bugging device. While we're sure that such things can be found on security sites on the web, we found this little circuit on the Poptronics page...