Hi Donald,
Thanks for your forum post to PDC!
My advice would be the Savi 700 product. It is a 3-line device which allows programmatic activation of all three lines from your application (PC, mobile and desk phone), it also facilitates connection to a wide range of deskphones through Plantronics EHS accessories (electronic hook switch).
The Spokes EZ Demo shows how to control (open/close/hold/resume) the 3 lines of the Savi 700.
I have some other sample code that actually demonstrates whisper prompts via PC line when there is an incoming call (including carefully tuned timers to wait for wireless link to headset to be established before playing whisper prompt). I will try to adapt the code to our current SDK and send it over...
Note: NAudio is a great free API for playing sounds through different audio devices.
Thanks,
Lewis.