Regarding DMZ ...
Maybe it is just my configuration (Frontier Internet with a Netgear D220D ADSL2 + Model Router) but
I was never able to make or receive calls until I put the Obi into a DMZ. It remained stuck in the "configuring" stage. Once I put the Obi device into a DMZ and added port forwarding, rebooted the router and did a factory reset on the OBI, I was able to get it to work.
I wasted a lot of time making router changes then trying to re-add the OBI without doing the factory reset. For me it seems that anytime I make a change in the router that reboots the router, I need to delete the configuration from Obitalk, reset the Obi to factory defaults, and start at the beginning.