There are some old posts related to this at viewtopic.php?f=9&t=846
, don't know if they're useful or not.
It has been a while since I've looked at this, but as far as I know, the best way to work with a cell phone is over a bluetooth connection, with your computer acting like a headset. So when your phone sees your computer as a paired device, the phone needs to see a headset service available. Last I checked, Windows does not include this service in the bluetooth stack built into Windows, but you can replace the stack ... the Broadcom stack does include this headset service. You can download the Broadcom bluetooth stack at http://www.broadcom.com/support/bluetooth/update.php
. I would recommend reading through system requirements, and make sure you have no compatibility issues before installing it. I tried this myself a few years ago on a Windows XP laptop, and it worked really well.
Another option to consider is to use something like Skype if possible (on the desktop/laptop). When Skype is running, you can display Skype Options, and set the microphone to point at your sound card. (It should be labeled as 'Stereo Mix', or 'Wave Out'). So the Skype audio would be directed at your speakers as usual, but you can speak using NextUp Talker, since Skype is using your sound card as the microphone.
If you try the Bluetooth option I'd be curious to know how it goes.