Some people have difficulties with setting up the Plantronics Multipoint system with their deskphones. And I agree they probably could have made it better by creating it to hook up to the phone line, in between the wall and your phone, instead of creating the HL10 Handset Lifter.
The HL10 Handset Lifter is a little Bluetooth module that hooks under your handset and when you press the button on your Voyager it picks up your desk phone and allows you to talk. Personally I feel this is a bit clunky and could have been better implemented, but I really can't complain, it works really well.