External SIP extensions

nano /etc/asterisk/sip_nat.conf
nat=yes
externip=<your fixed external IP> or
;externhost=yourdns.com
localnet=192.168.1.0/255.255.255.0
externrefresh=10

(Ctrl-X>y>ENTER)

Also, when adding the external SIP extension in FreePBX, make sure to change the nat=never default in the configuration to nat=yes for the extension that will be external.