This is absolutely incorrect.
OBi devices have always been, and as of today, still are, emulating Google Chat clients. As of today, Google is still supporting XMPP for OBi devices, now that Obihai has modified their device firmware to securely authenticate with your Google account.
Since your OBi device is acting as a Chat client, you merely need to make one outbound telephone call using Google Chat from Gmail. After you do that, the Chat forwarding destination will be added to your Google Voice account, and you can simply convert your Google account back to using Hangouts again. The Hangouts and Chat services will happily coexist, because Hangouts is entirely independent from Chat. Any/all Hangouts clients you wish to use, will continue to independently make and receive phone calls using your Google Voice number, totally unrelated to the operation of your OBi device.
Oh, and if you are still whining about the demise of Gizmo5, years ago, get over it.