Internet Calling is not what T-Mobile calls WiFi Calling.
T-Mobile's WiFi Calling needs OS hooks to access the SIM card. So, unless these hooks are part of the OS and given to an app, T-Mobile's WiFi Calling will NOT happen on a Nexus Device.
Internet Calling is if you have a SIP provider. Some people have VOIP devices that have a SIP account. THIS is what Google is talking about it supports.
Nexus 5 works just like iphone. The nexus 5 gets its updates straight from google just like how iphone gets its update straight from apple. The carrier has no control over the software(the OS) on Nexus 5 so t-mobile is not able to add wifi calling feature on this phone.
If you want wifi calling, buy a t-mobile branded phone(the one that has t-mobile logo on either front or back of the phone). t-mobile has control over those phones software update so they are able to add wifi calling. but with those phones the system updates can take upto 6 months to arrive on your phone after the new android version comes out.
Educate yourself, before you act like a pre-teen having a hissy fit (unless you ARE a pre-teen, then go right ahead).
Internet Calling is for SIP providers. T-Mobile WiFi calling is an IMS calling protocol. No need for me to explain it because an adult would educate themself before acting like an e-thug towards a large corporation.