T-Mobile uses technology that reads the SIM for Wi-Fi calling. They're not
going to do a SIP method for security to keep people from trying to use the
same number across multiple devices.
I hope this link works
I haven't tried this, but I was just poking around and found it
I just got my Nexus 5 and was wondering about the Wifi calling ability
If anyone tries it, please post back here and let us know of your results