I sometimes find it helps to put in an IP address instead of the host name.

Also try fiddling around with some of the network settings of the software. There should be a space to set up your IP addresses manually, and enter the IP address of your router/firewall.

Some providers take some messing with in softphones, but eventually they do work.

Eyebeam, in particular, has problems with resolving host names. Not sure if other X-Ten products have the same issues. Since this is not a registration problem with more a problem with the RTP transport, then perhaps you should go about checking your port forwarding. Software firewalls can also be an issue for softphones. Make sure your softphone is being permitted full acccess to the WAN.