You do mean T-mobile's version of "Wi-Fi Calling," yes? Just be aware that this is not traditional Sip service where you can bring your own Voip provider. This is just a quick way to enable T-mobile's voip service if you don't have cell phone service in your location, but you do have high speed Internet. It still uses your cell phone minutes, it's useless over the 4G network because that would be dead in the same area.
To enable your own Voip service you need apps. Sip Droid and Csipsimple are two I'm checking out but haven't great success so far. Not simple at all, and apparently Google Voice hates them. But admittedly I haven't tried very hard because I had to buy cell minutes to get the Android. (I'm using Sipgate BTW and like them it a lot on my Nokia.)
When is Android going to give us native Sip settings like Nokia's Symbian? It's not difficut at all. Methinks the wireless networks have talked them out of it. Hmph!