You may have been asking about access specifically through Hangouts because you don't want to use your carrier's voice system or you have no carrier voice service. You didn't describe your particular scenario and the problem you're trying to solve (the use case) so we're just guessing here.
As Steve said, you can't use the Hangouts app to call your own GV number. But you
can use Google Voice and your carrier's voice network connection to dial "from" your GV to your GV number, which will ring your home phones connected to the OBiHAI. This is handy when you're at the supermarket and you want to call home to ask someone to check to see if you need to pick up milk.
The setup is a trade-off. You have to go to the advanced settings for your mobile phone in
http://www.google.com/voice and choose "Voicemail Acces = NO." That means that when you dial your own GV number to pick up voicemail messages on your mobile phone you won't be taken directly to the VM sysetm. Since your house phones will ring, someone may pick up when you don't actually want them to. It also means that VM access will be more cumbersome because now you have to press the star (*) key during your outgoing message and enter your pin to get to the VM system. Possible, but not as convenient.
So it comes down to which is more useful/likely for you: quick, direct access to your GV voicemail -OR- the ability to "call yourself" and ring other phones at your house, work, etc.
This isn't an issue for me personally because I have GV forward my voicemails to my Gmail inbox and I listen to most of them there. Less frequently, I open the Google Voice app on my mobile phone and listen to my VMs that way. Even less frequently, I listen to my VMs by logging onto the GV site on my computer's browser.
