Here the schematic diagram of home telephone FM transmitter. This circuit connects in series with your home phone line and delivers the phone conversation through the FM band any time you pick up the telephone handset. Transmitted signal could be tuned by any FM receiver. The circuit features an "On Air" LED indicator and also gives you a switch that can be utilized to turn off the transmitter. A special characteristic of the circuit is the fact that no battery is required to operate the circuit because electrical power is taken from your phone line.
The transmitter circuit works by using only a short piece of wire aerial about 4" / 10 cm long to transmit the signal and a portion of the RF signal can also be radiated via the phone line itself. The circuit may possibly be implemented to share or record conversations, but will not be meant for illegal use.
Source: Home Telephone FM Transmitter Circuit