For a specific Directory Number (DN), although Call Forward Busy (CFB) is set to direct calls to voice mail, calls take a long time to transfer to voice mail when a user is busy.
The reason for this is that CFB does not get triggered if any of the following conditions are set. Instead, the calls get transferred after Call Forward No Answer (CFNA) is triggered.
Basic things to check for in this case are:
By default, Call Waiting is enabled system-wide. Depending on your requirements, Call Waiting can be turned off on individual phones or system-wide in Cisco CallManager.
Use the following procedure to disable Call Waiting system-wide.
Use the following procedure to disable Call Waiting on individual phones.
For further information, refer to Cannot Transfer Second Incoming Call.