From understanding the BTU that is in use has the ability to work as a VLAN untagging switch.

If TM is so paranoid about letting users gain access to these BTU which you want them to remain as your termination property, why can't you just untagged the ports on the BTU specifically for internet, TV, and phone allowing users to plug their own HGW, devices and phones to use on them?

Saves you the effort to supply crappy HGW router that requires VLAN untagging  and so much complications.

As far as I know, the latest deployments of FTTH in Vietnam etc only supplies a single unit of ONT that comes with 4 port switch/wireless and FXS VoIP phone ports built into a single box.

Can someone explain why?

If router spoil, I'll go out and buy a new one to replace instantly.

Phone still works because it's connected to the BTU(ONT or VDSL2 modem) FXS analogue RJ-11 port?

Consumers should be given the freedom to do what they want beyond the BTU ports?