I'm experiencing the same thing. I have the OBi202 w/ GV. I ported the number from Frontier to an old T-Mobile cell phone and then ported into GV. I don't know if porting has anything to do with this issue.
My number is 503-649-XXXX. The first number that I couldn't call was a bank. 503-601-XXXX and this was a few days ago before the firmware upgrade. Then today, I couldn't call a friend at 503-524-XXXX. So, I don't think the firmware is the problem.
I have Frontier FioS for my internet. And I'm using the OBiWiFi. A few people from the other thread on the 503 error suggested getting a dial tone, then waiting 7-8 seconds before dialing the number. I'm going to try that if I get the error again and will post the results.
Some folks suggested dialing 1 first on the GV forum. I'll try this as well.