Did you use the Obitalk web service to configure your OBi? Or did you find out the IP address of your OBi and configure it directly? If doing this on your own I would recommend the web service.
I just read the Belkin user manual for that router.
It's not the worst I've read. It does have a few glaring errors that probably will not hurt anyone.
I also noticed that Belkin lists only V1 firmware for that router. Apparently they still suffer from the problem they did several years ago when I last bought Belkin gear -- firmware does not get updated like it should.
I did not find any reference to a SIP "ALG" or "Application Gateway" function. That's not too rare, but they also don't mention FTP and not having that seems odd.
If you can find any reference in your router's web user interface, typically in the firewall section, to a SIP gateway, change the setting -- off it is on, on if it is off. If it makes no difference, leave it off.
If after trying this, and the stun server, your next step would be Callcentric support. They will probably have you reset your OBi back to factory defaults and configure it directly. I seem to recall a help page on the Callcentric site with instructions on how to do that.