Hi:
Well, not enough information to tell for sure. Either you don't actually have your own Google Voice phone number, and you're actually using a Google Voice "Lite" account, or else you do have a GV number, and you neglected to check the box to the left of Google Chat.
Log onto your Google Voice account from a computer's web browser, not from a mobile device, and go to this page:
https://www.google.com/voice#phonesLook on the left side of the web page. Do you see
the exact two words "Your number", and underneath it is the number you are trying to use for inbound calls, or, do you see the words "Access number"? If the latter, then this is Lite account, which is only used for voicemail. If that's the case, then you need to click "Get a Voice number" to get your own dedicated inbound GV number. If you
do See "Your number", then you need to have Google Chat listed as a forwarding destination, and you must have a check mark next to it.
Otherwise, if you do have your own GV number, and other people can call it and your OBi-attached phone rings, then all you need to do is go to the GV settings page, click the "Edit" button under Google Chat, and select "No" for voicemail direct access to have your OBi-attached phone ring (so somebody can answer it) when you call from your forwarding cell phone number.