sip.conf (snippet)

[general]
port = 5060
bindaddr = 0.0.0.0
context = default
disallow = all
allow = g729
allow = gsm

[Accela]
disallow=all
allow=g729
type=friend
host=64.124.221.10
dtmfmode=rfc2833
canreinvite=no
context=default
insecure=very

[1025]
disallow=all
allow=ulaw
allow=alaw
type=friend
username=1025
secret=1025
host=dynamic
dtmfmode=rfc2833
qualify=yes
mailbox=1025
nat=yes
canreinvite=no
insecure=very

sip_nat.conf
nat=yes 
externip=121.97.79.53
localnet=192.168.1.0/255.255.255.0 
canreinvite=no