Busy tone for 2nd call on Rainbow

clients are asking me that they would like to remove camp-on from their rainbow.

rainbow is connected through a remote extension in an oxe.

Scenario :

Extension number with rainbow is 1504.

Caller A calls extension 1504 and user answers from rainbow application

Caller B calls same extension 1504 and gets a normal ringing tone as if the user is not busy. Clients are asking so that Caller B gets a busy tone.

Regards

Wallace