Hi Dana,
not sure if this will work for you but here goes. I got the same error until I configured NAT on my router to Set up a Port Forwarding range entry for ports 5060 to 8050.
Then in "settings" in ExpressTalk, I ticked the box
"use static IP address and static mapped ports (Advanced Option)"
and entered in my public IP address ( your router should tell you this)
in the next box (External SIP Port) I entered 5060 and in the third (External RTP port starting from) I entered 8000.
Hope this works.
mhayes[at]associatemedia.com 