I'm sure it's possible but do you really want to do that, considering that ANYONE who called your number would get the same three options, and would therefore be able to make calls on your nickle? Once word got around you could have the most popular number in town!
However, all is not lost. If you go to the device's web page, and go to Voice Services, Auto Attendant, you will see that there is a "UsePIN" option and places where you can set up to four PIN codes. I have not tried to use that feature, but I suspect that if you set that up, then the AA would prompt you for a PIN before you could call anyone. If you can configure that and get it working to the point that you are comfortable exposing it to the world, then all you should need to do is go to Physical Interfaces, LINE Port and change the InboundCallRoute to: aa
Please understand that I haven't actually tried any of this, so I'm basically just speculating on how it should work. But it seems straightforward enough, as long as you test to make sure that just anyone dialing your number can't make calls. I'd use a strong PIN if I were you (at least 7 or 8 digits, and NOT your current phone number or anything that anyone other than you might be able to guess).
Keep in mind that you may not be able to use Auto-Provisioning if you do the above (at least until they have the upcoming "Advanced" section of the OBiTALK portal ready), since it might change some of your settings.