On the client side of OCS in Communicator there are 5 different TelephonyModes that you could set, either manually in the registry or via GPO´s.
If this value aren’t correct in there might be some problems for users when they try to call and a call might result in a "this calling feature is disabled" message.
To check the setting open regedit (or configure a GPO) and go to
Then check for a TelephonyMode setting
Set it to either
0 = Enable computer-to-computer calling only. No call control is enabled (default).
1 = Enable enterprise voice telephony features.
2 = Enable RCC and computer-to-computer calling.
3 = Enable both enterprise voice and RCC.
4 = Enable RCC without computer-to-computer calling.
5 = IM and Presence only; no audio