and Huawei HG850a?? everything expect WiFi
Nope you cant do anything to HG850a... the modem is blocking end user from accessing it.
As for as this forum concern, MicroTik still unsuccessful with the VoIP. Unless you can proof me wrong.
Ok, this is how I use my WRT54GL router with Maxis FTTH if you have Huawei HG850a GPON device. I assume it's mainly for landed properties.
Basically I followed all the instructions in this guide by rizanvp:
http://unifi.athena.my/mikrotik_rb250gs.phpThe difference is on the VLAN tagging, which is done as below:

(click for larger version)
Maxis has a lot VLAN's used, but the main ones should be:
621 Home Fiber Internet
822 VOIP
823 IPTV (apparently)
Possible to define more than those listed, but not sure how to go about it for now.
Following that, the port assignment. This assumes that;
Port 1 = Huawei HG850a
Port 2 = Your router
Port 3 = VOIP device
Port 4 = IPTV.

(click for larger version)
After setting that, connect your own custom router as usual and enjoy


Gotta love those delicious bandwidth graphs in Tomato!
TODO: Find out how to use VoIP in this setup. Unfortunately, my router is not a SIP device, but I assume if you have a SIP-enabled device, you can tag VLAN 621 and 822 on the same port to allow SIP connection.