I am bringing up this matter again to provide a new element which might help understand this behaviour.
First of all, I am having the same problem described here, and the trick of holding and the un-holding the call works for me as well. I have tried port-forwarding, firewall-disabling, all the way to making the PC an "exposed host" (like if it were directly connected to the Internet), all to no avail.
The new element is the following: X_Lite used to work without a hitch on a different network, along with 4 others PC using X-Lite similarly without problems. When moved the the new network, without any configuration change, X-Lite wasn't able to register anymore, giving a 403 error (forbidden). The only way to make it register was to change from automatic stun discover to "use specified server" (with a setting of stun.xten.com, but others worked, too).
I am happy with pressing hold twice when connected to that specific network, but I would love to understand WHY.