Until a call is connected (i.e ringing begins on the other end), everything you hear is generated by the Obi device - dial tones, busy signal, reorder, intercept messages, etc. That doesn't mean the Obi is causing the error - it is programmed to play certain sounds when it receives certain codes from providers. This is how telephony has long worked - your local central office would do the same if you were calling the other side of the country in the POTS world.
The Obi initiates the call via Google Voice. A hardware issue would be all or nothing, a configuration or software issue could be intermittent, but wouldn't discriminate against a certain number. You could always removed the GV line and then re-added it to ensure it's not a configuration issue. (If you changed any settings under "Expert" go into the corresponding ITSP and SP profiles and ensure the "ObiTalk" check box is checked on every line).
It truly sounds like a GV issue, which may include the signaling to your Obi, which triggers the intercept.