The OBi expects the PSTN line to be connected to its LINE Port and all telephones to be connected to the OBi PHONE Port.
When a call comes into the PSTN line, the OBi must wait for the second ring in order to receive CallerID, which is transmitted between the first and second ring. If you have a telephone connected directly to the PSTN line, you're going to hear those rings unless you disable its ringer.