Google DID NOT implement everything for HD Voice. There is already another thread to this. It may support part of the protocol doesn't mean it can do it. T-Mobile open sourced a lot of things which Google hasn't fully implemented. Like WiFi calling, HD voice, and such. You need to talk to Google about why they didn't fully implemented these features.
These two posts are at least some confirmation that the Nexus 4 supports HD Voice:
The first link is from someone on the Three network in the UK, and the second link is from someone on the Rogers network in Canada. So it seems like the phone is capable of supporting it. My guess is that T-mobile just isn't making it happen for some reason.
As for making a list, I agree, the only way I can see the phones that support it is by checking the HD Voice box in the phone search. But my point is that will only show you the phones they currently sell. There will be some discontinued phones that should support HD Voice. Which is why they should list all capable phones on that support page I linked originally.
Oh, and shouldn't the HTC One be listed when you sort by HD Voice phones? I didn't see it there.
I have yet to hear HD Vocie from the Nexus 4, and HTC One. FYI if you call a different carrier it would still work if they supported it. I have friends that use Rogers, and Three and they also have Nexus devices. I get better call quality on my Note 2's. IF you search through Google Groups you will see various posts of it not working or asking if it does. This is a good rule of thumb. If you ask the manufacturer and they don't give you an answer expect it to be "it doesn't support it."
The fact you linked XDA gives me reason to believe they are using custom roms which may have enabled HD Voice.
We would love to have you join us here at T-Mobile! The document you referenced about HD Voice has not been updates. The phones that we currently have that support HD calling are as follows:
- Samsung Galaxy S 3
- Samsung Galaxy S 4
- Samsung Galaxy Note II
- HTC One S
- Nokia Astound
- Nokia Lumia 521
- Apple iPhone 5 - 16GB
- Apple iPhone 5 - 32GB
- Apple iPhone 5 - 64GB
- Blackberry Q10
If the phone you are bringing over uses Wideband Adaptive Multi-Rate (Wideband AMR or WB-AMR), the HD voice feature will work on our network.