As a follow-up to my previous replies, Aruba has enhanced ClientMatch so it does not move a client while a call is active. Precisely what I was saying should not happen a year ago! :) This enhancement was added in 6.4.3.1:
Multi-Media Sync-Up
Client Match offers a tighter integration with multiple media-aware ALGs to provide better call quality for
programs like Lync and Facetime. With Client Match’s ability to understand various media protocols, clients are not steered to different APs in the middle of an active media session. When the AP learns that the client is in a call, it will not attempt to steer the client to another AP until the controller indicates that the call has ended, allowing calls to run more smoothly without any disruptions to the
ongoing media flow.