Free circuit schematics - wiparat.com

The FM telephone circuit is built on a PC board that is so small it can easily be fitted inside the housing of a telephone making it an instant pseudo-speak earphone.
This FM circuit connects in series with telephone line, steals power from it, and transmit at both sides of the conversation to an FM radio [...]

With some modification, I create my own version of FM Telephone bug complete with stripboard layout.
Theortically, the frequency range is from 88MHz to 94MHz but you may experiment with the coil structure to tune it.
Source : http://leoricksimon.blogspot.com