Cause youre running former version of 3CX, im sure I havent to tell you about the NAT and forwarding things?
Well, I´m not a very erxperienced user of this cool 3CX stuff, but I faced a problem with registration too. In my case it was due to having more than one configured ethernet adapter (NIC).
I removed some virtual adapters and it worked for me. I read some posts that there are similar problems with multiple NICs or multiple IP adresses of one NIC.
My thread is here:
http://www.3cx.com/forums/cant-register-2-sipgate-lines-at-one-time-3905.html
Have fun!