Hi, I have been trying to do this, and I have made some progress on it. I am still trying to resolve the issue of no outgoing audio
For FreePBX

In Peer Details [U]

username=+603XXXXXXXX
type=friend
secret=XXXXXX
qualify=yes
port=5060
nat=no
insecure=invite,port
host=10.225.0.1
fromdomain=ims.tm.com.my
outboundproxy=10.225.0.1
outboundproxyport=5060
disallow=all
authuser=+603XXXXXXXX@ims.tm.com.my
context=from-trunk
canreinvite=no
allow=alaw,ulaw
For Incoming Tab

User Details: empty
Register String: +603xxxxxxxx@ims.tm.com.my:pass:+603xxxxxxxx@ims.tm.com.my@10.225.0.1/+603xxxxxxxx
The machine hosting FreePBX must have 2 network adapter.
eth1: connect to LAN network
eth2.  connect to VLAN400 (Prior configurations to be done in switch/router) I used Mikrotik. There are existing tutorials to show how to do configuration for IPTV (VLAN 600), just repeat the same for VLAN 400
The secret /pass is a 6-digit number which you can get by calling customer service. Can only be obtain from supervisor level and above. Their customer service will act pro, but they actually don't know what that is.
If you have one way audio issues

1. Ensure disable SIP ALG in router
2. Check your routing table in FreePBX host.
If the setting is successful, you are able to have multiple simultaneous calls