Thanks.
So: I just now tested adding one of my Google Voice test accounts to one of my OBi devices, and it worked properly. I'm wondering if it was just a momentary system outage when you tried it.
Try this: Using Google Chrome browser on a laptop/desktop computer, not on a smartphone or tablet, press Ctrl-Shift-N to open an "incognito" browsing window (or, on Firefox, press Ctrl-Shift-P).
Sign into only the one Google account that holds your Google Voice inbound number. Confirm that you see the number on the web page. Leave that window open. Press Ctrl-T to open another tab in that incognito/private browsing window. Log into OBiTALK. Click the 200 device. Delete any previous SP configuration, then click the desired SP1-SP4, and click the button to set up Google Voice. Follow the steps. Make sure that the pop-up shows the same Gmail account as in the other browser tab. Does that work?