Did perform a factory reset as well and then set-up Google Voice as SP1. Still the same intermittent one-way audio problems. No real opportunity to test with another internet connection but at this point, I'm leaning toward this being a Google Voice issue, especially since I've seen at least one other recent post chronicling the same exact issue I'm encountering. The problem only seems to present when calling AT&T wireless phone numbers. Landlines are mostly unaffected, however the Ooma echo test gives me silence, no echo of what I'm saying. Conversely, calling another echo test service does work just fine!
If anyone has the means to do so, please forward this issue on to the proper people at Google. Yes, it's a free service, that I know, but to say this is immensely frustrating would be an understatement.