I bought my Nexus 4 from T-Mobile, and I can tell you that wifi calling definitely does work. I used the phone for an entire week via GrooveIP (my other phone was on perma hotspot basically) while waiting for my ST sim card.
What you should do is port your number to Google Voice. Then no matter what plan you go with and whatever number they give you, just add it to your GV account as a forwarder.
What your talking about is completely different from t-mobiles wifi calling. Any android made in the last 3 years is capable of that...
I was looking for something seamless, so you dont even realize that it is there. My mom and sister will be using it, and i already get enough calls a week about why the wifi's arent working right, I can only imagine how much headache this would bring.
And I tried to go that route myself a while back. I thought the idea was awesome the have a phone that is only reliant on having an internet connection (so at the time I actually had 2 phones - one that was only wifi), and it was cool enough, but nothing I would really want to give to my mom. I personally stopped using my google voice number, and just use it for vm and stuff like that since they STILL dont support MMS, and the interface is terrible. If they could fix either one of those two things I would gladly port my main number over to GV, but i dont see that happening any time soon.
If that was the case, then we would all just be using ST or net10 or something like that, but im too scared to port my main number to them, and dont want to lose mms and deal with GV's interface, so im stuck.
Not sure why T-mo doesnt just make the app or whatever it is that they use for wifi calling available for everyone. Clearly they would be the ones to benefit the most out of it, and would make a TON of people less timid about switching from ATT or Verizon.
Im considering having her buy a t-mobile branded phone - but I've had her set up on stock jellybean for the past few months and its all she knows. I also planned on just passing my phones down to her since she is comfortable having a smartphone, but this would definitely not let me do that.
EDIT: also - how much did you pay for your nexus 4 from t-mo? I thought they usually have the same price as the play store - which is why i was surprised to see it listed as $509 off contract.