HELP MODIFYING A DECT HEADSET

[1scaram]

I need some help modifying a dect headset (eg Jabra, plantronics). I am not worried about what make of headset is used but I need to modify it in such a way that I can stream both parts of the conversation through a usb card on a pc. I have the usb card which has a 4 pin 3.5mm socket in it, so it is a matter of adding a cable from the headset and any other components required either inline or in the earpiece to transmit both sides of the conversation. Someone else had a go and the interference was unacceptable. So if an expert out there has the time to help I would be grateful - I have diagrams of what has been tried to date - happy to pay for this. thanks