We're suffering this problem at my parent's place.
Setup is:
a) ATA: SPA-2002 with STUN enabled (specifically Substitute VIA Addr & Send Resp To Src Port: Yes) plugged in router
b) Router: Belkin Pre-N Wireless router (symmetric NAT), anti-hacker "firewall" disabled but NAT still in place, SPA-2002 private IP in the Belkin DMZ
c) Internet: cable modem (1 Mbps up, 6 Mbps down)
The SPA-2002 is registered on line one with Voxalot. Can call out fine. Can only be called by 'phoning' the user@ipaddress directly. Calling
user@voxalot.com fails (caller hears long pause then a fast busy signal). This seems to be caused by the Belkin. This problem is not observed on my very similar setup at a different location with a full cone NAT D-Link router.