Google also introduced some new features such as support for multiple SIM cards, Device Protection and HD voice on compatible devices with the update. The multiple SIM support seems to be primarily targeted at Android One devices. Google last month released Android One smartphones in Indonesia running Android Lollipop build 5.1 out-of-the-box.
Another highlight is built-in Device Protection, which makes sure the device is locked when lost or stolen, until signed-in using a Google account. Interestingly, Device Protection once enabled requires a Google account sign-in even if someone resets the device to factory settings.
The Android 5.1 Lollipop update also brings high definition voice calling on the mobile phone. The HD voice calling will be available only on compatible Android 5.1 devices. Google gives example of HD voice on the Nexus 6 via T-Mobile or Verizon. Other features include ability to join Wi-Fi networks and Bluetooth paired device option directly available from Quick Settings.
Soon after announcing the Android 5.1 update, Google started pushing Android 5.1 factory images for Nexus devices. At the time of writing, the company posted factory images for the Nexus 5, Nexus 7 2012, and Nexus 10. We can expect the company to release factory images of more Nexus devices in the coming days.
Google, apart from factory images, also started uploading source code to the Android Open Source Project (AOSP). First reported by Android Police, currently Google only posted the source code of Nexus 5, Nexus 6, and Nexus 9. The publication notes, "Branches with the name lollipop-mr1-release [maintenance release 1] are starting to appear under an assortment of different projects."