You can narrow down your troubleshooting:
Google Voice doesn't use SIP, so ALG is irrelevant. (At least temporarily) remove Callcentric from the path: Log into your Google Voice account on a web browser. On the new Google Voice user interface, click the three horizontal bar "hamburger" icon on the left side, then scroll down and click "Legacy Google Voice". Click the gear icon in the upper right corner to get to Settings. On the Phones tab, add a check mark to the left of Google Chat, and remove the check mark next to your Callcentric number.
On the OBiTALK web portal, reconfigure your OBi to use your Google Voice SPx as the primary line for outbound calls, AND tell it to ring your phone on inbound calls.
The only thing you will lose is caller ID name. Try this configuration for a few days or a week, and see if call quality and reliability improve. If they do, then it's an issue with your router and/or SIP-ALG. If the same symptoms persist, then it's something else.