What do I do if my asterisk server has a private IP Address?
Please enter the following two settings (externip and localnet) at the top of your sip.conf file in the general section.
If public, nothing needs to be added to the general section. Depending on your setup, you might need to forward all ports
to your Asterisk server's private IP address or put the private IP address of the Asterisk server on the DMZ.
externip=WAN IP address
localnet=192.168.1.0/255.255.255.0 (or whatever your private LAN is configured for)