Cable for Siemens MT50

The described cable is intended for connection of mobile phone Siemens MT50 to HP200LX. It has many advantages: it's simple, cheap, compact and has low power consumption. It was tested with Siemens MT50 and S35 only, but it would work probably with similar phones (C55 etc.). Here is the curcuit diagram:

The electronics can be built-in into small home-made plug as demonstrate below: