For a specific directory number, although Call Forward Busy (CFB) is set to direct calls to either a telephone number or to voice mail, the calls do not get forwarded when the original directory number is busy.
Basic things to check for in this case are:
Find out if the directory number is shared. CFB does not work with shared directory numbers.
Check the Call Waiting field in the Directory Number Configuration page. This should be set to off.
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:
From Cisco CallManager, select Service > Service Parameters.
Locate the CallWaitingEnable parameter and change the drop-down selection to False.
At the top of the page, click Update.
Repeat the above steps for all Cisco CallManagers in a cluster.
Use the following procedure to disable Call Waiting on individual phones:
Search for the directory number (or phone).
Select the directory number at the left (if you searched by phone).
Locate the Call Waiting setting on the page and change the drop-down selection to Off.