If you have not already done so, add and verify a regular 10-digit US telephone number as a forwarding phone number on your Google Voice configuration. Then, unplug the OBi from power. Test that inbound calls to your Google Voice number ring the forwarding phone number and you can answer the call. Have your parents call your GV number. The expected behavior (of course) is that their call will ring through to your forwarding phone number. Does that work, or do their calls go directly to Google Voice voicemail?
If it works, there is nothing wrong with your Google Voice setup. If all other calls ring your forwarding number, but your parents' calls do not, then you have most likely accidentally marked their phone number as spam. When you mark a call in Google Voice's call history as spam, subsequent calls from that number will be sent straight to voicemail. Look in your Google Voice call history and find the call, and un-mark it as spam.
I'll also mention that I have never been a OOMA customer, but years ago, they had some sort of rather confusing method of integrating Google Voice with their service. I don't know if it still works or not, but, if your parents OOMA number has that feature turned on, I have no idea how it might impact their calls.