From the Microsoft announcement post:
In build news, today we are releasing Build 14393 for both PC and Mobile to Windows Insiders in the Fast ring. This build includes a handful of new fixes to take note of:
Improvements and fixes for Mobile
- We fixed an issue resulting in rapid battery drain when Visual Voicemail syncs voicemail messages on Dual SIM devices like the Lumia 950 XL.
- We also fixed an issue causing some battery drain on older devices like the Lumia 535, 640, 735, 830, 930 and Icon.
- We fixed the issue causing Voice Recorder to not show up consistently as an option for Call Recording. A app update was released last Friday that fixes this issue. Make sure you have the app version 10.1607.1931.0.
- We fixed several issues impacting Dual SIM capabilities. Using a device with two SIMs should work as it should.
Known issues for Mobile
- We are continuing to investigate W-Fi issues on certain devices.
- Turning off Bluetooth can sometimes result in freeze, crash, or reset. See this forum post for more details.
- Wallet users are prompted for a PIN twice when using tap to pay from a locked phone. You can enter their PIN twice and tap as usual once the phone is unlocked. A fix for this will come as a Wallet app update via the Store.
- REMINDER: We have changed the backup format for Windows 10 Mobile devices to reduce the size of the backup stored in OneDrive. As a result, if you do a backup on a device running the latest Windows 10 Mobile Insider Preview builds and move back to the released version of Windows 10 Mobile (Build 10586) and restore from your backup – your Start screen layout won’t restore and remain the default Start layout. Your previous backup also gets overwritten. If you need to go back to Build 10586 temporarily, once you are on Build 10586 you should disable backup so it doesn’t overwrite the good backup from Windows 10 Mobile Insider Preview builds. We will stop noting this going forward.
Certainly Wi-fi and Bluetooth issues should be enough to force another bug fix update for Insiders before the 'official' start to the rollout. My guess is that this will be delayed and/or staggered.
My devices are updating as I write this. I'll report back, no doubt! The last build hasn't been without problems on the editorial devices here, but hey, this is the 'fast' ring for Insiders, so what do you expect?! I do maintain at least one device on the 'slow' ring too, so that I can monitor progress. Plus even more phones on the production builds.
Media controls continue to annoy me, though. Comments welcome from you lot!
As ever, head into Settings/'Update & Security'/'Phone update' in order to check the download and install progress of the update on your Fast ring device....
As per all my recent charts and comments, note that Redstone builds are NOT available for the non-Win-10-approved phones (e.g. Lumia 920). no matter how many times you install the 'Windows Insider' utility and enthusiastically tap the 'Fast ring' entry.... The highest version of Windows 10, even unsupported, for older (mainly x20) handsets is 'Threshold' ('the 10586.xxx branch, still being offered for a few more weeks under the 'Release Preview' ring). See my feature on all this if you're unsure where your phone will end up in Microsoft's roadmap.