As many of you are finding out WiFI calling from TMobile is not available and won't be availble probably due to the fact that this is an unbranded phone.
However, I have found a fairly simple solution that works for me. I have a weak signal at my home so I only want WiFI calling while I am at home.
Here is what I did. I created a Google Voice account and setup a new Google Voice number.
From there the Google voice app in the Android Market does not allow WiFi calling. It only allows calls made over your carriers service. So I download GrooveIP Lite. It allows you to use your Google Voice account to make calls over WiFi. Now you will need to tell people that if you want them to call you over WiFi they need to call your Google Voice number. It may not be the most clean solution but it does work. I am not giving up my Nexus 4 just to get another phone that will do tmobile wifi calling.