iOS 9.3 is coming in the Spring and will introduce Night Shift as well as a number of other new features.
T-Mobile to Release Software Update With LTE and Visual Voicemail Support for Unlocked iPhones on April 5
The T-Mobile Carrier Update is a minor iOS software update that enables official iPhone support by T-Mobile. When installed, the software update enables a handful of capabilities like Visual Voicemail, MMS Settings and Network/Device optimizations that customers do not have access to today.
On April 5, the software update will begin being pushed via OTA to all iPhone devices on the T-Mobile network with iOS 6.1.x or higher.
The software update for existing handsets will come one week before T-Mobile officially begins offering the iPhone, although it has for a number of years catered to iPhone users seeking to bring their unlocked devices to the carrier.
While existing T-Mobile customers using unlocked iPhones will be able to access LTE speeds in just a handful of cities, the carrier is working on rapidly building out its LTE network. Existing users will also continue to have some limitations on network access, as Apple will be releasing a tweaked iPhone 5 as part of the T-Mobile launch, with the new hardware providing full compatibility with T-Mobile's network including AWS Band 4 frequencies that are unsupported by iPhone devices sold to date.
Band 4 is where much of T-Mobile's 3G network is housed, an issue that has long forced iPhone users on the carrier to fall back to slower EDGE networks on Band 2. T-Mobile has been working hard to shift its 3G network over to Band 2 to provide greater 3G compatibility for existing customers, but the transition is not yet complete.