You would think an Obi202 would have logic to guard against this, but apparently not...
Use * * * 6 to get the latest firmware (maybe not necessary, but I had done this recently).
Go into the device's web interface (or Obi Expert Configuration if you use their web configuration site) and go to Physical Interfaces, PHONE 1. Scroll down to call forwarding and enable ForwardOnBusy and make the ForwardOnBusyNumber ph2. This means if a call comes into Phone Port 1 and it is in use it will get forwarded to Port 2.
Now I think at this point all you have to do is busy out both phone ports and place a call to the number associated with Port 1, that will at least get you nothing but dead silence and probably other weirdness. But to REALLY burn the cake, go into Phone Port 2 and enable ForwardOnBusy and make the ForwardOnBusyNumber ph1. This means if a call comes into Phone Port 2 and it is in use it will get forwarded to Port 1. But if you busy out both phone ports (just taking them off hook is sufficient) and call into the number associated with port 1, not only will you get dead silence on your incoming call, but the OBi202 will completely lock up and go offline, and you will need to physically interrupt the power to the device to get it back.
Now, I realize that at a basic level I've created an endless loop, yet what I was attempting to do was something desirable - if a call comes into either phone port and it is busy, send it to the other one. In most cases where the potential for an endless loop exists, the programmers won't allow it. In this case, there should be some kind of internal indication that the call was already forwarded from port (1 or 2), so don't send it back there. Something along the internal equivalent of a SEEN-BY tag, for anyone old enough to remember Fidonet.
Then again, it's not like there's really good documentation on how to use these features. I just used raw ph1 and ph2 without any enclosing brackets or braces or parentheses; when I tried using a raw extension number that did not work at all (I probably needed to specify a service provider for the forwarded call and didn't recall how to do that offhand). The online docs I looked at were at http://www.obihai.com/OBiDeviceAdminGuide and from what I am seeing there, it appears braces are optional. I didn't feel like trying to repeat this test too often, for fear that if I did it once too often my OBi202 might not come back after a power cycle.
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