Verizon and AT&T both announced recently that they will introduce WiFi calling in early 2015. They will also both allow cross carrier VOLTE. As far as WiFi calling is concerned, I'm not sure if they will update existing firmware or just new firmware going forward. That will remain to be seen I suppose. Tmobile already has WiFi calling, and from personal experiences, it is awesome as long as you have a good net connection. They even sell a cell spot router to capitalize on the feature.
That's all the info that is widely available. There are other options out there, but involve getting a SIP carrier, VOIP apps, etc... Google Voice is part of this. Google VOIP, Google Voice and SIP for more info on how to use the Native dialer on your android phone to make VOIP calls over wifi.
I don't believe 5.0 will bring wifi calling unless the carrier and phone allow for it. You may be better off with the nexus 6 and their promise of wifi calling in 2015. Since you have tmobile, you will get this feature.
Good luck