T-Mobile doesn't give eta's on software updates or for that matter even confirm updates are coming, it all stems from issues when they did a while back and the mfr had issues with the update and of course people started suing everyone so now they don't say anything.
Typically T-Mobile updates come after the other carriers because Wi-Fi calling is not an app but has to be integrated into the OS so takes the manufacturers some extra time to program, T-Mobile some extra time to test and get approved by the FCC. Now if you look at the HTC One T-Mobile actually beat AT&T so it does vary sometimes.