I seem to be having the same problem with you. After I've connected from Archer C7 LAN port (for VoIP) (originally on 192.168.0.1) to the Maxis Thomson Router's WAN port (192.168.1.254), I was able to disable the Thomson WiFi, disconnect the eInternet connection, check the telephony settings. But the VoIP wasn't working and I disconnected from the Maxis Thomson router, now I cannot connect to it anymore on 192.168.1.254....

Sorry for my ignorance, but what do you mean "untag"? 

Apparently I am not allowed to leave the vlan id field blank, unless I flash it with an OpenWRT firmware 
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After I restarted the Maxis Thomson router I was able to connect into it again.
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There is no "Bridged HSI(GPON) + VOIP" option under the list of services in my model/version of Maxis Thomson router.
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My Archer C7 is defaulted to 192.168.0.1. Can someone advise if I need to change the IP of the Maxis Thomson router or disable its DHCP for the bridged VOIP to work?
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Apparently I am not allowed to leave the vlan id field blank, unless I flash it with an OpenWRT firmware 
[attachmentid=7338179]

I think I kinda understand what you are doing, as I was thinking to do something similar as well.

Original idea: Connect the TM ONU to my Archer C7 as the main router for WiFi/internet access. Then bridge from Archer C7 to Maxis Thomson to use the Maxis VoIP... no success so far.

Alternative: Connect the TM ONU to Maxis Thomson as the main wired router for internet access as well as VoIP. Connect Maxis Thomson to Archer C7 and use the Archer C7 as an AP for WiFi.

Will try it out to see.... fingers crossed.

there's no way u can ditch that Thompson if u want to use Maxis VOIP except if u change the ONU/BTU setting.

basically can....

-need to know the ONU IP
-login / password to control panel
-Maxis SIP details.
-know how to do VLAN tagging.

u good to go.

Well, tested and it works... VoIP works, internet works!!

So my setup is as below:
- Maxis Thomson remains as my main wired router for internet access as well as the VoIP dialer. Disable the WiFi of Thomson router.
- Convert the Archer C7 to a wireless AP (by setting its IP to be the same segment as Thomson, and disabling DHCP)
- Connect ethernet cable from Maxis Thomson LAN port to Archer C7 LAN port

However, I can't help but to feel that internet download/browsing speed seems to have suffered with this additional hop of an AP, but running a Speedtest when connected to the Archer C7 wireless AP and directly to the Maxis Thomson router seems to produce consistent results either way. So, I'm giving it a benefit of a doubt (maybe too many users are streaming Rio Olympics) and gonna continue monitoring the performance.