I can't speak to how the OBiTALK portal configures Google Voice on 508s, but on the 200/202 configuration,
obihai.sip.google.com is automatically configured, and is the correct value. During the beta, there were a couple of other values used, but this is now the "production" value.
Unrelated to that, although 508s can use Google Voice, it is not recommended. The free, consumer version of Google Voice is neither intended nor supported for business use. Google can and does shut down accounts for abuse if it becomes aware of calling patterns associated with call center or telesales, or any other sort of business use that involves high-volume outbound calling. If that happens, you lose all of your Google Voice phone numbers. Now that Google has a paid, commercial version of Google Voice for G Suite customers, it's a good bet that they will step up enforcement.
It's best to use a 508 with one or more paid ITSPs.