Problems with registering

I have asterisk on a pc with fix ip address When i register from a sofphone in the same computer, there is no problem, but when I try from a sofphone in a different net, it is not possible to register. why is that?
I also have the same problem when i try to registered from a mobile phone N80 over 3G, the register fails.
what is wrong?

Check the ip for which Asterisk is listening for sip connections (parameter bindaddr in sip.conf) and if a firewall is running.


Marco Bruni