I found dat internal call waiting is not functioning on phone's when All Busy Mode for SIP Forking is enabled.
I have 1 user with 2 phone's and no call waiting, This extension must give a busy when one of the phone's is in use.
But I have another extension, there is call waiting enabled, but give a busy as there is a second call to this extension.
At the moment I disable "All Busy Mode for SIP Forking" everithing is oke for the user with call waiting.