I replied in another thread that the MWI signal is sent by the telephone company to which your endpoint is connected. So, if you're using a SIP VoIP ITSP, that ITSP has no way to know that there is a message waiting on your GV account, to toggle MWI. It can only manage MWI for its own VM servers. When you used Google Chat to connect to GV, the OBi was getting Google's MWI signal; now it isn't. So, no way to do what you want. Instead, use the Google Voice mobile app on an Android or iPhone, and/or configure GV to send you a text message when you have a new message waiting.
RE: accessing VM, this is a bug with your SIP ITSP, not Google Voice. You should be able to correctly spoof your GV caller ID on any ITSP that allows it, and then dial into your GV number, which will see the caller ID, and let you into your VM. I've done this with Callcentric, Skype, the Android and iOS Vonage app, and several other services. Some ITSPs aren't implementing caller ID spoofing correctly, (it needs to be spoofed all the way out their terminating PSTN LEC, not just via SIP), and so it isn't working for their customers. It might also be something you've done wrong on your OBi. As a test, why don't you unplug your OBi, then provision your SIP ITSP's credentials on a softphone (I use Counterpath X-Lite or Bria), and see if it works. If it doesn't then it's your ITSP's fault. If it does, it is something wrong with your OBi configuration.