Many T-Mobile users rely on Wi-Fi calling to use their mobile number without needing access to the mobile network, but they aren't Nexus 5 owners. That handset, due to its lack of carrier modifications, didn't support T-Mobile's nifty feature. With the Nexus 6, things will be different. The carrier has already announced that it is bringing Wi-Fi calling to the handset.
T-Mobile says it will deliver Wi-Fi calling via an over-the-air update at some point in early 2015, thanks, it says, to enhancements in Android 5.0.