One Lync Subscriber may have multiple devices.
A feature is required that enables established calls to be "moved" from one device to another while in mid-call. This would enable users to take a call on their mobile device (VoIP or Cellular), walk into their office and continue the call on their PC Client.
Of course the reverse should be possible as well. with the call moving to the moble device over VoIP or Cellular.
The other participant(s) in the call should be unaware the call has been moved from one device to another.