That's a good question zigzagthrust. Since it's still not a T-Mobile branded phone, we can't guarantee the Wi-Fi Calling will work. I may be optimistic, but I've heard of some folks being able to use service like this on their phones. If you've already got the phone unlocked, the best bet would be to connect to a Wi-Fi network and give it a shot.
You'll need to change your apn settings to tmobile's from metropcs's while using tmo sim card if you want wifi calling and volte with a supported device. Google tmobile apn settings and you'll find the settings, just add them to your apn's and you should be good to go.