Assuming we're talking about a Google Voice account that's configured on an OBi...
Forwarding incoming calls to (or calling) your own Google Voice number from an OBi results in checking your Google Voice voicemail, not leaving a voicemail.
You would need aother service provider to use for forwarding calls out of an OBi to your own Google Voice number if you want Google Voice to ultimately answer that call (after 25 seconds of ringing) and accept a voicemail.