IF government will shutdown even Internet, will it keep land-line phones online?

You can start from linmodem project: http://bellard.org/linmodem.html , it may allow to create 9600 connection on 1GHz processor.

Of course, phones have built-in DSP processor, but each phone model has it own DSP. Some OpenGL chips are able to do FFT using vDSP (e.g. iPhone4), so it can be used to accelerate soft modem.