Whether you want to share your phone with a family member or better manage your mobile costs, Android Lollipop 5.1 now lets you use more than one SIM card on a device with multiple SIM slots.
With Device Protection, your lost or stolen device will remain locked until you sign in with your Google account - even if someone resets your device to factory settings. This feature will be available on most Android phones and tablets shipped with Android 5.1 in addition to Nexus 6 and Nexus 9.
High Definition voice calling is now on your mobile phone. Get crystal clear calling with HD Voice between compatible Android 5.1 devices like the Nexus 6 on T-Mobile or Verizon.
Android 5.1 Lollipop also provides enhancements such as the ability to join Wi-Fi networks and control of your paired Bluetooth devices directly from Quick Settings.
[Source: Android Blog]