Right now it seems that there is no way to stop anyone from dialing international calls from Polycom CX600 phones owned by privileged users when those users aren't at their desk and forgot to sign out.
This has been an issue for a long time: http://social.technet.microsoft.com/Forums/en-US/ocsclients/thread/2c764ac1-6df2-4692-b974-c3f913bfbe16
Is this still not fixed in Lync 2013? I've tried setting DisableHandsetOnLockedMachine but this doesn't seem to work. Also, I think 'Machine' in that case refers to the PC which the phone is plugged into. Is there a way to get this to work with a non-tethered phone?