I have a windows XP SP3 build with Lync 2010 on it as well as Cisco IP Communicator 8.X
The user has a Jabra Bizz 620 headset which was "certified" for Lync and Cisco IP communicator.
As long as Lync has the Jabra set as its primary device when the user tries to make a Lync audio call it also unhooks the line on the Cisco IP communicator. So the Lync call will go through but you will hear the dial tone in the background because IP communicator is off hook. If you hang up in Cisco IP communicator it will also hang up the call in Lync.
If you set the primary audio device to the laptop speakers in Lync then if will not off hook Cisco IP communicator. It does not matter what audio device is set for IP communicator, as long as Lync is set to use the head set it off-hooks both programs.
The user also has a polycom speaker phone that will do the same thing if it is set as the primary audio device in Lync.
I know this may not be specific to Lync but trying to cover all bases. If anyone has any wise words I would very much appreciate it.