According to the Obi Admin Guide the default is NO. I looked on mine and didn't find a setting to turn on. But there is a star code for BargeIN. *96 I guess pressing *96 turns it on.
EnableLINEPortBargeIn
This option is useful only when a PSTN line is connected to the OBi110. By default a user can pick up the phone and dial # to connect to the LINE port. If no one else using the PSTN line at that time, the user will hear dial tone from the phone company. On the other hand, if the line is already in use, the user will hear fast busy tone if this option is disabled, or join the current call in progress otherwise.
This option is only available in OBi110. Default setting = No
EnablePHONEPortBargeIn
Enable the caller to barge in when he calls the other phone port from this phone port while the other phone port has an active call in progress, on-hold, or ringing.
This option is only available in OBi110. Default = yes