It sounds like the OBi is disconnecting the PSTN Line port based on an extended period of silence instead of detecting a disconnect signal such as a CPC (momentary loss of voltage) signal, a disconnect tone, or a polarity reversal issued by your PBX.
I would try to determine if that is in fact the case and if the PBX is issuing any analog signal on the line to signal disconnection and if so why the OBi is not detecting it. The signal goes by different names in different parts of the world. Calling Party Control or CPC, Disconnect Tone, Disconnect Clear Time, Disconnect Supervision, Open Loop Detection are some of the terms used in conjunction with the signal.
The OBi has a number of parameters you can work with under Physical Interfaces-->LINE Port-->PSTN Disconnect Detection.