bigtoodog

add the followings into /etc/hosts:
;203.80.89.135        s2hkbntel.net     s21.hkbntel.net
203.80.89.139         s2hkbntel.net     s22.hkbntel.net
;203.80.89.146        s2hkbntel.net     s23.hkbntel.net
;203.80.89.150        s2hkbntel.net     s24.hkbntel.net

Trunk->Outgoing Setting->Peer Details:
type=peer
defaultuser=12345678hk
fromuser=12345678hk
secret=passwd
host=s2hkbntel.net   
fromdomain=s2hkbntel.net   
canreinvite=no
insecure=port,invite
qualify=yes
sendrpid=yes
;nat=force_rport,comedia
port=5060
context=from-trunk
dtmfmode=auto
disallow=all
allow=g729&ulaw&alaw

Trunk->Incoming Settings->User Details:
username=12345678hk
type=user
host=s22.hkbntel.net
fromdomain=s2hkbntel.net
context=from-trunk
outboundproxy=s22.hkbntel.net

Trunk->Registration->Registration String:
12345678hk:passwd@s2hkbntel.net/12345678