What sort of message? What do you mean, exactly? Why?
It sounds like you may not understand how Google Voice works with OBi devices.
GV is not simply a free telephone carrier used by OBi boxes. It's an inbound call forwarding system. Someone calls your GV number. GV then forwards that call to your list of forwarding destinations. There can be a total of six, in any combination of US mobile or land line phone numbers. One of those destinations can also be Google Chat. OBi boxes are pretending to be Google Chat clients. When you answer a call on your OBi-connected telephone, you are answering a call forwarded to the OBi device's chat client, by Google Voice.
So, the caller is either going to get you, live, or they'll get your GV voice mail greeting. The greeting is recorded and managed by you, on your Google Voice account, here:
https://www.google.com/voice#voicemailsettingsGV can also provide a "call screening" feature. If you turn it on, you, the person answering the call, will hear "call from Joe Blow", if you have a Google Contact listing for Joe, or if you've had GV ask Joe to record his name previously. If not, you'll hear "call from unknown caller". In either case, you will then be asked to press 1 to accept Joe's call, or to press 2 to send Joe to GV VM.
https://www.google.com/voice#callsettingsThere are some unrelated OBi features regarding OBi's own "auto attendant" feature, but again, it's unclear what you mean, and what you want to do. Describe your desired behavior in more detail...give a hypothetical example.