Lot of time we need to disable audio/video telephony in the Lync client as per business requirement. This is particularly useful where you want to deploy Lync in a terminal services/session host environment, and don’t want users to be able to answer audio/video calls. The result in the Lync client is the removal of any audio/video settings or actions. Additionally inbound audio video call notifications will not be receive.
To disable telephony the following registry key needs to be added:
Reg Add "HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Office\15.0\Lync" /V "TelephonyMode" /D 5 /T REG_DWORD /F
Note: You can also add this to HKEY_CURRENT_USER
You can also do this manually in regedit. You will first need to create the keys Office -> 15.0 -> Lync, then a DWORD with the following values