Beats me, since 404 is about as ambiguous as error 500. As you know, it's just saying "I tried to connect to this address, and it didn't respond back". I think that is a symptom/byproduct of whatever your OBi is trying to reach at that point being the wrong destination.
I'm still wondering why it reports that particular (wrong) IP address in the failure. I wonder if it's a DNS issue? You said that your non-GV SIP Service Provider still works. Does the OBiTALK dashboard show the device as "Offline", or does it just show the two GV SPs as not connected? You could try going into expert mode and setting the DNS servers to Google's DNS, at 8.8.8.8 / 8.8.4.4, or the Quad9 DNS at 9.9.9.9 (
https://www.quad9.net/). Note that there are two different places to set DNS in OBi Expert, depending on whether you are using Ethernet or WiFi.
I know you're an expert, but assuming nothing, did you try power-cycling your router to clear out its cache?
What makes troubleshooting problems with Google Voice unique, is that, since there are so many users, if something is actually failing on Google's side, we quickly see many, many reports of the same error on the GV forum. Nobody is reporting the error you're getting.
I have a couple of test GV accounts that I know work, since they're currently active on my OBi phones. If you'd like to rule out an account-related issue, PM me and I'd be happy to let you borrow one of the accounts.