An adapter and signal transmission cables for hands-free telephone system including a mobile telephone, car audio equipment such as a cassette recorder and a mobile telephone adapter. The mobile telephone adapter includes an adapter body, which can be pushed inside the cassette recorder; a hands-free microphone; a first signal transmission cable for connecting the adapter body and the mobile telephone; and a second signal transmission cable for connecting the hands-free microphone and the mobile telephone. The adapter body has a housing that resembles a cassette tape externally and a magnetic recording head installed inside. One end of the first and second signal transmission cables are joined together and connected to a plug, and the plug is capable of plugging into the voice input/output socket of the mobile telephone. When the mobile telephone picks up incoming voice signals, the signals will be transmitted via the adapter to the cassette recorder, which is then broadcast out through its loudspeakers. Similarly, any out-going voice signals will be collected by the hands-free microphone, and then passed on via the adapter to the mobile telephone for broadcasting. Therefore, a bi-directional communication is established.