There is not enough information in your post to diagnose the problem, since it could have one of several different issues.
The first thing to check is whether or not you actually authorized/connected the OBi 200 to the correct Google/Gmail/Google Voice account. It is possible to connect to an account that has no Google Voice phone number, and thus, no number to which calls can be forwarded.
There are two easy ways to determine this. Call some other number from your OBi-attached telephone. Look at the numeric caller ID on the called party's phone. Is it your Google Voice inbound phone number? If not, you are connected to the wrong Google account.
Next, open a web browser on a desktop/laptop computer, not on a phone or tablet. If you're using Google Chrome browser or Safari, press Ctrl-Shift-N to open an "incognito" browsing window, or, if you're using Firefox, press Ctrl-Shift-P to open a "private browsing" window. In that window sign into the one and only Gmail account that holds your Google Voice number. Go here:
https://voice.google.com/settings. You should see your Google Voice number at the top of the page. Now, leaving that incognito/private window open, press Ctrl-T to open another browser tab. Sign into the OBiTALK web portal here:
http://www.obitalk.com/obinet/pg/obhdev. Click your device, then click the SP1-SP4 configuration that holds your Google Voice connection. Find the Gmail address it is using and compare it to the one in the Google Voice tab. Is it the same?