Stream windows audio over network and send back mic sound

Question

I am a little lost as to how I go about implementing this kind of setup...

We use SIP Softphones in our office, I would like to be able to, from another computer on the network, listen into the call live and speak to the person on the phone without the other person on the call hearing me...

I have looked into this in detail and have found 3 solutions...

1. Whisper function on asterix pbx (Our supplier does not support this)

2. icecase (having no joy getting this to work at all, and wouldnt support the ability to talk to agent anyway)

3. Team viewer ( This works but only alows a VOIP call between manager and sales person, doesn't actually allow me to hear the client on the phone )

Any help would be brilliant.

Thanks