I just read in the news this morning that T-Mobile has enabled Adaptive Multi-Rate Wide Band (AMR-WB) functionality on its GSM cellphone network here in the USA--in short, HD Voice is now active on T-Mobile. They said that the Samsung Galaxy S III and HTC One S will support it "out of the box" now, and any cellphone T-Mobile sells that supports AMR-WB will have full HD Voice functionality.
I believe that the iPhone 5 does support AMR-WB, and since T-Mobile has said they may offer the iPhone 5 by late spring 2013, that could mean any iPhone 5 running "officially" on T-Mobile will support HD Voice, which means a huge leap up in call sound quality.