I can't believe this thread. There are more than 50 posts, yet not a single person has taken even a modest effort to troubleshoot the problem!
It's clear that many thousands of OBi customers, the vast majority, enjoy clear phone calls. However, there is also compelling evidence that in some circumstances, the OBi is less robust than other ATAs or software, resulting in no or poor quality outgoing audio.
If we can accurately report what goes wrong, then Obihai can find the bug or weakness and fix it. In the meantime, the details will likely suggest a workaround to mitigate the problem.
This isn't rocket science. The OBi might be improperly coding the audio, sending corrupted or improperly timed RTP, having intermittent networking issues, or triggering a problem at the provider. Or, maybe it's some other issue I haven't thought of.
As a start, do you have the problem with multiple providers, e.g. Callcentric as well as GV? On both incoming and outgoing calls? When you are reasonably certain that the problem is not on the other end, e.g. calling a corded phone connected to a landline? Does it affect bridged calls, as well as calls from the Phone port? During a bad call, can you make a concurrent good call (using a softphone or other VoIP device)? Via the same provider and server? To the same destination?
Next, take a look at what the OBi is sending. Using a dumb hub or a PC with two NICs, set up your PC so it can "see" the OBi's traffic. Capture the packets with Wireshark. When you have a bad call, save the RTP in a file and listen to it. If bad, look at the payloads for clues. If good, check the stream for timing or other technical problems. If there are none, check whether e.g. a concurrent ping to the provider's server shows severe jitter or packet loss.
Of course, I have no idea what may be going wrong here; my specific suggestions may be inappropriate for the problem at hand. However, keep your eyes open and think about what may be happening. Pretend you're a doctor and determine what's wrong with the patient. Don't just report that he's sick.