It's sad that the state of hardware and firmware engineering can be so sloppy that we assume the worst about functionality...I am often surprised when things "just work", to quote the old Apple slogan.
In this case, things do just work as you wish. When an OBi 2000 series phone has the screen saver (or more accurately, the "turn off the LCD") feature enabled, and someone calls and leaves a voicemail message, the big red message waiting bar will illuminate, and the LCD will of course wake up. After the timeout, the LCD will go dark, but the VMWI (Visual Message Waiting Indicator) will remain lit.
Now for the caveat: VMWI is a feature that depends on the telephone service provider to turn on or off. It also depends on you, the user, enabling it. Assuming that it's enabled on your phone (if you use the OBiTALK portal's pre-configured service provider templates, it should be), and assuming your service provider supports it, it just works.
Google Voice does not support VMWI, so it can't control the indicator. It depends on you, to set up notifications to your mobile phone or email, when you get a message.