Due to many ppl pm me to ask for the access into HG850a, here is it..
If you're using Maxis FTTH, this setup will enable you to plug the telephone into the HG850a, use custom router and you can put your Thompson router into store room or maybe rubbish bin.  
You can go straight to the end of this post to see the final outcome.

Do it at your own risk.

Preparation

1. Download & install autohotkey http://l.autohotkey.net/AutoHotkey_L_Install.exe
2. After finish install autohotkey, launch it and right click on the AutoHotkey icon at the right corner taskbar, click on \"edit this script\", a notepad will open. Change the content to following:

3. save it and close the notepad, right click the AutoHotkey icon again and click \"reload this script\"
4. download putty http://tartarus.org/~simon/putty-snapshots/x86/putty.exe
5. configure your pc ip to 192.168.100.123 subnet 255.255.255.0
6. Switch off the HG850a and plug LAN cable connect ur pc to the HG850a LAN 2

Action
1. open a command prompt [windows key + R, type ' cmd ', enter]
2. type to perform continuous ping to 192.168.100.1
3. open putty client and ready for telnet: input 192.168.100.1 and choose telnet. Dont click 'Open' Yet.
http://i.imgur.com/HOv4EYV.png 

4. Switch on ur HG850a and check the continuous ping
5. Once you saw you're getting Reply from 192.168.100.1.. IMMEDIATELY click \"Open\" in the putty client.
6. You're now successfully telnet into the HG850a but dont be happy, your access will soon be block once the HG850a finish loading the iptables, so you need to perform the following steps quick and accurate.
7. login by 
username: root
password: admin

8. press the hotkey [ ctrl + alt + z ] , wait for the command to finish typing, 
then immediately repeat pressing [ctrl + alt + x], let the command finish typing between the interval.This is the Hotkey that we configure into the AutoHotkey previously. This is to let the AutoHotkey help us to type the command in a very short time, before the iptables kicks in and repeatedly wipe the iptables.

9. After around 10 seconds, if you still get response from the telnet (respond to your \"enter\"; press enter can go next line), then you're safe for now.. but not permanent, if u power off and power on your HG850a, you will need to do from step 1 again.

Configure to have permanent access and no need to hassle so much, perform the following:

Web Interface
1. Open your browser, go into http://192.168.100.1
2. Login with admin account:
Username: telecomadmin
Password: hs5711Bbfh
3. go to 'maintain' > 'device' > 'configure file' > 'Download Configuration File' to backup the config.


4. after download, make a duplicate copy of your config so that if anything goes wrong you still have the original config.
5. open the 'downloadconfigfile.conf' using notepad
6. ctrl+f to find

7. change that line to 

8. save it and go to the HG850a web interface, go to 'maintain' > 'device' > 'configure file' > 'choose file' > select this modified config > 'Upload Configuration File'
9. After finish restore, power off and on the HG850a, you should now able to access the web interface and telnet even after power cycle.

Setup VoIP into HG850a [Maxis]
1. Go to 'Basic' > 'WAN' and you should now see by default '1_VOIP_R_VID_400' {not too sure about the name but I know there is a default there}
2. click on it to select, change the VLAN ID to 822. Change and save it as below


3. Go to Status > 'Device' > 'WAN' you should see it connected with an ip address


4. Go to 'Basic' > 'VoIP' and setup the SIP as follow


5. Go to 'Status' > 'VoIP' , you should see the status 'Registered' and the 'voip' led on the HG850a should lid up.


6. Plug your telephone into 'Tel 1' Port and try to make a call to your mobile phone.

Setup custom router
Any router
Refer to bro thankyou!
vlan capable router


Final look..


Credit: Thanks MrSpeedy for giving me some important information which originated from yjwong. 

Anybody know how to prevent TM from reconfiguring our HG850a? Out of sudden, my voip went dead and I couldn't seems to get into the HG850a from the URL. So, did the whole process again and found that my old voip is now in place instead of the maxis voip. It seems that TM has pushed their config down to my modem again. Grrr...

I'm facing the same problem.  Have to reconfigure everything back to default again.
I think to prevent them setting back to default is by changing the telnet password and web password?  Anybody dare to try?

I did that, won't help, they re-provisioned the modem now and then. Somebody said on every reboot.

I accidently restore default the terminal without saving the configuration file. now i cannot connect to internet.please help me how to configure this terminal from beginning just like unifi did.Or can someone send me the config file?


image uploading site

Preparation

1. Download & install autohotkey http://l.autohotkey.net/AutoHotkey_L_Install.exe
2. After finish install autohotkey, launch it and right click on the AutoHotkey icon at the right corner taskbar, click on "edit this script", a notepad will open. Change the content to following:


3. save it and close the notepad, right click the AutoHotkey icon again and click "reload this script"
4. download putty http://tartarus.org/~simon/putty-snapshots/x86/putty.exe
5. configure your pc ip to 192.168.100.123 subnet 255.255.255.0

Action
1. open a command prompt [windows key + R, type ' cmd ', enter]
2. type
to perform continuous ping to 192.168.100.1
3. open putty client and ready for telnet: input 192.168.100.1 and choose telnet. Dont click 'Open' Yet.

5. Once you saw you're getting Reply from 192.168.100.1.. IMMEDIATELY click "Open" in the putty client.
6. You're now successfully telnet into the HG850a but dont be happy, your access will soon be block once the HG850a finish loading the iptables, so you need to perform the following steps quick and accurate.
7. login by
username: root
password: admin

8. press the hotkey [ ctrl + alt + z ] , wait for the command to finish typing,
then immediately repeat pressing [ctrl + alt + x], let the command finish typing between the interval.This is the Hotkey that we configure into the AutoHotkey previously. This is to let the AutoHotkey help us to type the command in a very short time, before the iptables kicks in and repeatedly wipe the iptables.

9. After around 10 seconds, if you still get response from the telnet (respond to your "enter"; press enter can go next line), then you're safe for now.. but not permanent, if u power off and power on your HG850a, you will need to do from step 1 again.

Configure to have permanent access and no need to hassle so much, perform the following:

Web Interface
1. Open your browser, go into http://192.168.100.1
2. Login with admin account:
Username: telecomadmin
Password: hs5711Bbfh

Anybody know how to prevent TM from reconfiguring our HG850a? Out of sudden, my voip went dead and I couldn't seems to get into the HG850a from the URL. So, did the whole process again and found that my old voip is now in place instead of the maxis voip. It seems that TM has pushed their config down to my modem again. Grrr...

I'm facing the same problem.  Have to reconfigure everything back to default again.
I think to prevent them setting back to default is by changing the telnet password and web password?  Anybody dare to try?

eeyak
4. Go to 'System Tools' > 'Configuration File' > 'Save Configuration', once complete you are now safe

Safe meaning if the modem is rebooted, no need to start from step 1
Coz can access via http ?

So to put the long story short, I got a Cisco SPA122 ATA to handle the VOIP connection. Plugged into my switch and so far after working out some configuration hitches with a kind soul, no problems now. 

Anyone successfully configured WR841ND load with dd-wrt to works with Maxis Fibre ?
Especially on VLAN part...
Quanta any comment ?

Try KLseet's setting https://klseet.com/index.php/setup-walk-thr...setup-for-unifi
Vlantag 500 replace with 621. All the best!

Tried but failed... VOIP also dunno how to configure...
At the end flash openwrt again, now 621 n 822 working like charm...

Been awhile since I last check in here. Initially I had no problems with the VOIP after ditching the Thomson and connecting it directly to the HG850a. It was until late last year that I suddenly had no access VOIP. Thought it was a problem on Maxis side but they reported nothing. But I swear they changed something to make it not working. And the post above probably confirms it. Was too lazy to bring out the ladder and connect the laptop to reconfigure the HG850a again since it's located on a server rack high up.

So to put the long story short, I got a Cisco SPA122 ATA to handle the VOIP connection. Plugged into my switch and so far after working out some configuration hitches with a kind soul, no problems now. 

I believe this is the best solution for now, I've an old Linksys (now CISCO) ATA which is not being used right now. I've SIP account with Manchester UK call area, probably I'll try to power it up again.

Where can buy Ciso SPA122 ATA or newer router with VOIP connection.
This will be the ideal replacement for Maxis Thompson router.