Information about call protection Busy on Busy

In ProCall Enterprise, you can configure the settings for "Reject calls when busy" in the Settings – Telephony – Call protection section. Information on this can be found in the context-sensitive estos ProCall Enterprise Client Help (accessed via the F1 key) and under ProCall Enterprise Online Help.

Depending on the configuration of the telephone system, you can have second calls delivered on "Call waiting" and set Busy on Busy for one line or across all devices. 

Call state (alert state)

Incoming calls, incoming audio/video chats, or incoming screen sharing requests are not logged until the call or request is answered.

From when is the participant considered occupied?

The call protection Busy on Busy and cross-device Busy on Busy status is only activated when the call or request has actually been accepted and a connection has thus been established. 
Only then is the participant considered to be busy for the purposes of call protection.