The outside Simultaneous Talk and Receive callbox does not have the ability to receive calls from the internal base station. In normal operation the outside callbox must first call in to the inside base station so the call can be answered. An optical sensor can be used to automatically make the call if you do not want the user to press a button.
This firmware change allows the outside unit to auto-answer the call from the inside station. Pressing the Talk button on the inside base station will automatically connect you to the outside unit so a conversation can take place without any action required from the outside callbox.
This feature will not work if there are multiple outdoor callboxes.