Maybe it's the Auto Campon Timer instead. I noticed in help: "This option applies only to sets without softkeys, ONS stations, and trunks calls."
Entire Description:
Enter the length of time while listening to busy tone before system performs an auto camp-on to the busy line. Values range from 0 to 30 seconds. A blank field turns the timer off and provides busy tone to a caller when calling a busy extension.
This option applies only to sets without softkeys, ONS stations, and trunks calls. Sets with softkeys do not require auto camp-on since they provide a "Wait" soft key that gives the user control over camp-on.
Note: The timer should be disabled (field blanked) for MLPP installations and SIP phones.
Note: Setting the Auto Camp-on Timer to 0 seconds can result in callers not getting a busy signal when they encounter a busy internal line. Set a longer time, e.g. 10 seconds, to ensure that the caller will receive the busy signal.