T-Mobile has started to roll out a new beta program called Digits. In many ways it's a next generation FindMe, Follow Me type service that goes somewhat farther than GoogleVoice.
While I've yet to try it, what seems the most appealing to me is the ability for a call to be placed or received on an iPad or Android tablet as it looks like what T-Mobile is doing is some marrying up of IMS and VoLTE to make that happen.
I've pinged T-Mobile's PR team to try to get more clarity, and once I've enrolled in the beta I'll provide more insight. For now, the Verge has most of the facts. The most glaring is how T-Mobile wants you to disable iMessage.