Sorry to jump in, but the port forwarding issue has been on the table for some time now..

Every provider has different routes and different upstream carriers. Depending on how those carriers config their systems will depend on whether you need port forwarding or not.
For a long time with my Spa2102 ATA, I didn't need it. With my PAP2T and HT502 it seems to work better if I have the full range 5000-65000 enabled.
At the same time, I think the routers are pretty much the first line culprits...Many of them seem to do their own things no matter how they're set up..I've never had to use STUN.
My ISP provides dynamic DNS so my routher is set to DHCP, however, I point the forwarding route to a static IP imposed on my ATAs.

I come from the POTS telephony world and VOIP is still a constant learning process for me. Don't know if I'll ever learn.