If you just want a very basic unit, without a crystal, then there are numerous example RF circuits on the net, try searching for them. You can modify an "FM bug" circuit with a DTMF transmitter to modulate it, and likewise, an FM receiver withe a DTMF receiver at the other end. IT WILL BE ILLEGAL TO DO THIS IN MOST COUNTRIES WITH ANY DEGREE OF RF SPECTRAL CONTROL, save under the circumstances in some countries that you are a qualified radio amateur, and transmit limited power in certain bands.

Steve