Let's first use the correct, unambiguous terminology, so we can understand what you mean.
You have a Google Voice inbound phone number. You (should have) added one or more linked, forwarding phone number(s) to your Google Voice configuration. When someone calls your Google Voice phone number, GV will forward that call to your linked/forwarding phone numbers, and to your OBi. OBi devices act as Google Chat clients, and so they are simply one more forwarding destination.
Are you saying that a) you linked your mobile phone number as a forwarding phone, and b) when you call the mobile phone number from the OBi-attached phone, you don't hear ringback tones in the OBi-attached phone's earpiece while the mobile phone is ringing?
If not, please clarify. If that IS what is happening, then try calling some other phone number from the OBi-attached phone. If possible, try calling numbers on several different phone carriers. What, exactly, happens?
