Hi, Finally I can confirm that Maxis VoIP can work on Yealink IP Phone

This part of my project user replaceable device
1. Ditch ONU, use GPON SFP on Business Grade Router
2. Ditch Router, use Maxis 4G LTE Backup on other device
3. Ditch Router, use Maxis VoIP on other device This Thread

This should work on any Maxis on any fiber, however, I tested on TM-Maxis.

TLDR:
Phone ID / Username:
Your Phone ID and Username, it should be same


Password:
Your password, you can find just do Account Number + Last 4 Digit phone number


SIP Server:
Note: This FQDN routable via Proxy, it will resolved IP Address to 14.192.234.109


Proxy Server:
Note: This FQDN routable only on Maxis Network (PPPoE, 4G, VLAN), it will resolved IP Address to 202.75.146.26


Codec:
Note: using unsupported codec result no incoming or outgoing call


Register Expires (sec):


Register Retry Interval (sec):


DTMF:


User-Agent:
Note: Maxis VoIP only accept valid User-Agent sub-string, without this, you will get Error 405: Method Not Allowed


Waiting User-Agent for these model, if anyone willing to help, I appreciate it, use WireShark to find it:


As you can see, I highlighting User-Agent in RED! This is the reason your VoIP not work on other place, SIP Server looking specific sub-string before get Authenticated.

Tested Device:
Yealink T21 E2
Configure like this, simple is better
[attachmentid=11095319]

You need change Yealink User-Agent via CFG file, Export, Edit, Import!
[attachmentid=11095291]
[attachmentid=11095301]

NOTE: Yealink custom User-Agent will append like this, not total replace, it may not work in the future.
[attachmentid=11095314]

Tested SoftPhone:
MicroSIP (Windows)
Open MicroSIP and add 1 account like this, then close MicroSIP
[attachmentid=11095315]

Then edit microsip.ini find userAgent= and paste it, like this:
[attachmentid=11095316]

Then open MicroSIP, try call:
[attachmentid=11095320]

Android / iOS
So far I didn't find suitable SoftPhone that allow custom User-Agent, if you find one, let me know~

ONU POTS
I tried on HG8240H and HG8240H5, not working even with custom User-Agent, apparently TM OMCI override VLAN 822 IPoE, you can try if TM OMCI not strict as mine here...

Accepted Network
1. VoIP VLAN
2. Maxis PPPoE Internet
3. Maxis 4G/LTE

I have tired on non-Maxis (TIME, Unifi, ) it seem FQDN sgb-cvx1.146.26 are not reachable

SIP ALG
You can use VoIP without VLAN, however, your router must support SIP ALG.
Ubiquiti EdgeRouter having trouble with SIP ALG, you must use VoIP VLAN

You mean Analogue Telephone Adapter?
What I discovered, Yealink just IP/SIP Phone like others

then, you need modify Linksys User-Agent.

or, install 3cx PABX, (middle man), surely any ATA can work, as long 3cx sending valid User-Agent
Instead of:
Maxis VoIP > ATA

Do like this:
Maxis VoIP > 3cx > ATA

I dont have Yealink, it was my friend, we tried it,

another way to troubleshoot, adjust your microSIP User-Agent like this:


if your microSIP work with that User-Agent, check Yealink config