I want high quality sound from the telephone for making long distance
coaching calls. So I want to use a binaural headset and microphone. I
could get a seperate headset like the Plantronics ones, but it seems
to me that I could just link the telephone to my Mac computer and
listen and record (I'd like to record too) etc from the computer,
using the headset and microphone I use for Skype already.

Do I need an interface, or is the line in and line out to the
telephone just normal 2 volts type line level? In other words I could
make a DIY interconnect with just phono plugs for the computer line
in/
line out sockets?

All help welcome for this. I can solder connections and am handy with
sound equipment.

andy