I've a very weird problem.
We are testing X-Lite 4 without no problems in a LAN, and 3CX softphone in the same computer is working too (both are using same credentials to register).
Additionaly, we've published the SIP server to Internet to make some external test.
3CX is working without problems (just changing LAN private IP address and using WAN public IP address), but X-Lite is impossible to register.
We've checked again in our LAN and everything is working fine.
I think there is some bad configuration in our X-Lite when we are using externally.
Our SIP Server, is Avaya IP Office 500.
Thanks for your help!!!