We had this issue with using a Sipura behind a Belkin Pre-N router (that fails the WinSTUN test). It also made direct URI calling between SIP devices impossible, although many other things, such as DIDs mapped to
usernumber@voxalot.com and calling out to the PSTN from the Sipura worked fine. I think the correction on our end was to disable the 'extra' firewall protection in the router, and put the ATA in the router's DMZ, in addition to the usual stuff of having your ATA correctly configured for STUN. I don't know if that allowed the FWD echo test to work, but doing that did allow direct URI calling to
username@dynamicip.net. It is clearly a router issue.