> I have tested with SjPhone and then with a wifi SIP/GSM dual phone
> called 'Tc-300 from T-COM ' (http://tinyurl.com/2rt94q)

I set mine up using the instructions here:
http://www.frikis.net/tc-300-voipbus...as-gratis/566/
- works fine here with no port forwarding or any fiddling on the
router config.

Spex