Google android announced on its official blog about this update that they are Unwrapping a new Lollipop update which is Android 5.1 . This update offers a few new features like support for multiple SIM cards, Device Protection and high definition (HD) voice on compatible phones and one more, it offers you use more than one SIM card on a device with multiple SIM slots.
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.
It also provides enhancements such as the ability to join Wi-Fi networks and control of your paired Bluetooth devices directly from Quick Settings.