One would think that Obihai would maintain the linked post to list the current firmware release, but the person who was acting as "forum custodian" is no longer in that position, and the wording in the post is outdated. In fact, you're running the correct, current firmware version and the post is just incorrect as to the number.
The symptom you describe could be caused by anything related to the reliability of your home network connection and possible hardware failures in your modem, router or OBi device. I assume you meant that it does work for a while, or sometimes, until it stops working. This suggests that your Google Voice and your OBi configurations are correct.
Have you observed any other problems with your home network, such as streaming video or music stopping, or a computer that seems to hang momentarily while waiting to load webpages? It's going to be very hard to diagnose this via the forum; you may need to take your OBi to another location to at least rule it out as the failing component.