This is a Robot car which can be controlled using any mobile phone. This was a project I did years ago, while learning digital electronics in the college. It was a time when I knew only less about microcontroller programming else almost the whole circuit could be replaced by a small microcontroller. The  mobile phone is placed inside the car. To control the car, just call that mobile phone  in the car after enabling the DTMF keypad tones in the calling phone. Then press 2 to move forward, 8 for backward, 2/4 for left/right control.The following video shows it in action although it is not controlled by another phone in the video.

The mobile phone kept in the car is to be kept in automatic answering mode. It will cause the phone to automatically attend an incoming call. After the call is attended, one can easily control the car by through his phone.  The circuit of the car is shown below:

The circuit consists of DTMF decoder MT8870 which decodes the DTMF tones into the corresponding BCD codes. They are then decoded by the common 74154 4line to 16 line decoders. Its output is active low, so it is inverted using 7404 Hex inverters. Then a small logic is implemented by the 7432 OR gates to decide the direction of the motors , it then controls the motor driver L293D. See some construction pictures :