How to filter out ringing current in a local battery telephone system?
Simply coupling the signal via a small capacitor will act as a high-pass filter, however the output waveform of a magneto is very peaky and has harmonics that could be hard to filter out without affecting the audio. 

To provide complete protection you should put back-to-back Zener diodes across the signal wires wires to clamp voltage spikes, and a resistor in series to limit current. Usually a transformer is also inserted to isolate the computer's ground and provide a balanced output to the phone line.

Here's an example of a passive phone line interface (it has two identical circuits for input and output - you only need one of them).

Computer audio interface to wire-based telephone network