A virtually undocumented feature in the OBi is Barge In.
If a call is placed from one OBi to another and the called OBi's PHONE Port is already in use, the called party will hear a Call Waiting tone. The called party can choose to ignore the new call or, the current call can be placed on hold and the new call answered by pressing the FLASH button on the phone.
If the calling party first dials *96, however, and waits for a new dialtone before calling another OBi whose PHONE Port may already be in use, the called party will hear a Barge In tone intead, just before the calling party is joined with the current call.