The devices use the same hardware. Their firmware is different. The 20x series support Google Voice. The 30x series do not. The 30x series are generally intended for resale by Internet Telephone Service Providers (ITSPs), who will typically pre-program them to use that provider's service. If you see one for sale on eBay, don't buy it.
No, you are not smarter than Obihai, and no, you can't magically load the 20x firmware onto a 30x device. Just buy a 20x device, if you want to use Google Voice.