So, to be clear, when an outbound call doesn't work, what, exactly, does it do? Do you just hear endless ring tones, or a busy signal, or dead air? When this happens, if you hang up and try again, does it work (without rebooting the OBi)? And, when this happens, does the system status always show "connected"?
Given that the problem persists between two different GV accounts, and your OBi is at the current firmware level, I have to suspect either a hardware problem, or something fishy with your network setup. You described a "default DMZ". The OBi should work perfectly fine, being connected on a standard port, not in any sort of DMZ. If you have closed off any ports or otherwise tinkered with the router's firewall settings, that could explain the issue.
If you could take the OBi somewhere else, like a friend or family member's house, or otherwise plug it into some other network, that would be another troubleshooting step. Another approach would be to temporarily disconnect your home network router from the cable modem, and plug the OBi's WAN port directly into the cable modem for testing. This assumes that you can release the DHCP lease your router has on the cable provider's network first, before detaching it, and that you have the OBi's own router function enabled, and then allow the OBi to obtain a lease (otherwise, your cable provider might block the second MAC address trying to get a lease).
