trixbox IP public

I install Trixbox on server, the first I set IP static for trixbox, I register x-lite with trixbox, is ok. but after that, I change into IP public, I can't register (408: request time). I don't know why?
please help me. thank all
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Firewall,Port
Check the firewall on sip port or port you are using for sip registration with x-lite when you use public ip