Both T-Mobile and Sony have been a little slow to roll out Lollipop to the Sony device owners on the network, and while the Sony Xperia Z3 T-Mobile model sits quietly, patiently waiting for the update to appear when other models have already received the update, it appears that T-Mobile customers who are still using the Xperia Z1s (T-Mobile's variant of the original Sony Xperia Z1) are starting to receive the Android Lollipop update on their devices. Since this is a Sony device it will likely not be able to pull down the software update over the air and install it, and much like the Sony Xperia Z3 unlocked models which have been on Lollipop for at least a month, it will probably have to be connected to a computer and installed through USB.
While this is a bit strange to see an older flagship of Sony's receiving the Lollipop update before the Sony Xperia Z3, perhaps this means T-Mobile is really close to sending out the Xperia Z3 update. They had just recently listed the Xperia Z3 as transitioned into the T-Mobile testing phase from Manufacturer development, so the actual update could be very near especially now that a model two generations prior is receiving the software.
According to Xperia Blog the update software is listed as Android 5.0.2, which is no surprise since this is the version other Xperia devices who have been updated to Lollipop are currently on. The build number for the new software is 14.5.B.0.236 and is for the Xperia Z1s model number C6916. Users can wait for the update to come to their device over-the-air but should they want to bypass the waiting period as there is no telling how long it will take to roll out completely, Xperia Blog has a link to the FTF firmware to be downloaded and flashed manually to the devices. If you aren't sure on the process for flashing firmware, it's highly recommended that you wait for the update to hit your device on its own time, which shouldn't be too long now that the process seems to have begun.