asterisk users - Feb 2016 - Phone audio sound routing through workstation audio ports

Hi,
so this is not Asterisk related maybe, only in so much, that one of the systems
involved is a SIP PABX running on one...

There are actually 2 parallel running systems on my site: one is SIP based, with
hardphones and softphones; and the other is Siemens HiPath.

Agents are currently using headsets connected through splitters to the HiPath
phones,
and the SIP hardphones only have handsets.

Juggling that during office situations is a hassle.

Because the agents computers run SIP Softphones 
(Microsip, soon to be tSIP SIP Client because, wow, here BLF is possible, 
in a free software program, anyway... and it's not overly flash ;) )

and they are bound to use computers while call is on anyway, I thought of
entirely
ditching the hardphones.

Instead I'd like to connect the PC rear input/output as the other handset to
the HiPath through splitter,
(yes it'll require adjusting line-output of pc to mic-in of the phone, but
that's not that difficult)
and don the headsets to the front PC input/output instead. 

So when call comes on HiPath, they can lift, and press a button on screen to
have voice.
Or a slider.

But when call comes over from the softphone, they 'lift' and hear it on
headsets (maybe after switching the slider over, no matter).

So the question is: does a SIMPLE configurable PC Audio router program exist ?
That works at a push of a button?
and is not a full-blown mixer?

Any keywords (I found JACK so far but it's a library mostly) I should try ?

Any hints would also be appreciated.

Kind Regards
el es