Is this an old-fashioned concept? What is the point of call waiting - disable?
Please bear in mind that the use of GV on the OBi110 is an unofficial supported product that is not sanction by Google. Google Voice was not design to be used as a telephone system per se. The Obi ATA are designed as a generic ATA to be used by SIP among other things. GV has two channels and if one is in used and another calls comes in on the other channel and is not answered it will go to GV Voicemail. This has nothing to do with the OBi ATA. I think the call waiting feature works as advertise and the second incoming calls does not ring your phone or presents any type of call waiting tones.
I'm curious though as to why you do not want the second call to go to voice mail? Why is this so critical to you that the second caller gets a busy signal? This is a weird scenario if you ask me.....