My understanding is that T-Mobile does a proprietary GSM-voice-over-IP protocol. Every Android phone you get from T-Mobile has this baked in, but I don't believe it's available on stock Nexus phones. Typical discussion thread. The standard workaround seems to be to install a third-party VoIP client. If you're using a VoIP client that connects to Google Voice and your friends are calling you on that number, then everything seems to work, except you can't roam seamlessly from WiFi to cellular and back like the proper T-Mobile WiFi calling stuff.

Edit: It's not actually proprietary. It's a standard. T-Mobile apparently licenses the relevant code from Kineto.

See also, detailed Verge article.