In order for OBi device to pass PSTN Caller-ID to the PHONE Port, it needs to completely decode the caller-ID signal prior to ringing the phone. The ring delay is to make sure OBi has enough time to do this. This is also needed in order for OBi to perform caller-ID based inbound call routing. For example, you might not even want to ring the phone for certain callers.
For US, Caller-ID is sent shortly after the end of the first ring. So potentially you can adjust this delay parameter to about 3000 ms (which is counted from the beginning of the first ring). If that works for you reliably, you can stick with it. The default value is conservatively set to 5500ms to make it work for most PSTN services out of the box.
The delay parameter can be found on Physical-Interface->Line Port->Ring Delay
On the issue of picking up the phone on your PSTN line does not immediately stop the ringing on the OBi phone:
Are you using a cordless phone attached to the OBi? If so, it is the cordless phone issue. You might want to try connecting the cordless phone in parallel with your regular phone on the PSTN line, I suspect it would behave the same way.