Hmm, you mention that your router is "designed for SIP" -- that may be a clue. I had a router some years back that had some internal settings for SIP, that, when ENabled, caused external SIP clients (softphone or ATA) to NOT work. So, those router settings had to do with the router itself supporting SIP. Have you looked for and disabled any such settings? It sounds counter-intuitive, but it's worth trying.
I agree with Ron that the router needs to be eliminated as the culprit; a worthwhile test would be to put your integrated DSL modem/router into bridge mode (just use it as a DSL modem and temporarily disable the router section), and let the OBi grab an IP address directly from your ISP's DHCP server (if that is supported by your ISP), or else borrow a Linksys or D-Link router, plug that into your DSL box, plug the OBi into the router, and see if that fixes it.
I have never had to mess with port forwarding, firewall settings, etc, to make VoIP work, when the router is working properly. I have a friend who had a Actiontec integrated DSL modem/router combo that never worked with SIP, either. Sometimes the telcos that provide these integrated modem/router boxes have the configurations altered to intentionally disable things like VoIP, although you said your PAP2T worked, so this is a mystery.
One last thing: have you tried the OBi/Netopia combo with a ITSP other than Google? It would be helpful to know if it's limited to Google vs. another provider. Sipgate, for example, is free and works reliably enough to use as a benchmark. Same goes for a basic/free Callcentric account.