Two weeks after the previous build (15031) we have a brand new Windows 10 Mobile version, build 15043, for all those on the 'fast' Insiders ring - highlights are a barrage of bug fixes, especially to Bluetooth, and some extra functionality for reading ebooks in Edge.

From the Windows blog, here's what's new (I've emboldened items I mention below):

What’s new in Build 15042

Improved reading experience with Microsoft Edge: You loved the new Edge reading experience and gave us a TON of feedback. Here are some we implemented:

• When reading a locally saved EPUB book in Microsoft Edge on PC, a book icon will now be used in the place of the previous generic in the tab bar.

• If you switch pages while an EPUB book is being read aloud in Microsoft Edge, the reader will now jump to that new location in the book.
• If you’ve changed the read aloud settings when reading an EPUB in Microsoft Edge, those settings will now be preserved for subsequently opened books.

Other changes, improvements, and fixes for Mobile

• We fixed the issue causing the Settings app to crash when navigating to Settings > Devices and you should be able to pair a Bluetooth device without any issue. The Bluetooth quick action from Action Center should also now work again. This issue could have also resulted in decreased Flashlight quick action reliability.
• Using Continuum to connect to a display wirelessly should also work as expected.
• We fixed an issue resulting in screen rotation sometimes unexpectedly not working on the last Insider flight.
• We fixed an issue where live tiles were not updating some apps such as Outlook Mail and Calendar.
• We fixed an issue where under Settings > Update & security > Phone update Insiders might see the text “Some Settings are managed by your organization” even though the phone wasn’t being managed by an organization.
• We fixed an issue from recent flights where you couldn’t paste or drag a folder from the PC to your phone
• We fixed an issue where tapping the link for a website with an associated app would crash Microsoft Edge and not open the associated app.
• We fixed an issue where downloads from Microsoft Edge directly to an SD card might unexpectedly use space on the local phone storage as well.
• We fixed an issue where the keyboard might dismiss after a second when opening a new tab in Microsoft Edge and quickly starting to type.
• We’ve fixed an issue that could result in your personal typing dictionary being unexpectedly deleted and having to train the keyboard from scratch.
• We fixed an issue where, during the download of a phone update, after disconnecting then reconnecting to Wi-Fi due to low reception you might get an error saying the download needs a better connection to continue, despite currently being connected to a good Wi-Fi connection.
• We’ve improved the video playback experience when using Continuum for phone wirelessly over Miracast.
• The icon for Windows Insider Program under Settings > Update & security should be no longer a square.

Known issues for Mobile

• Speech packs will be unable to download on this build.
• Adding a new card / paying with an existing card will not work in Microsoft Wallet.

The items emboldened  are bugs that I'd run into personally. And presumably just about every other Fast ring Insider - this is a major build in terms of reliability and stability, I think. The screen rotation issue alone was driving me absolutely insane...

How many builds will we get before Redstone 2 will finally 'ship' to production phones? My guess is another three, taking us to the last week in March, following which we'll have the official release and then some 'Release Preview' builds to tidy final bugs up through April/May.

Which means that at some point soon after the Creators Update hitting, many Fast ring Insiders here might like to consider switching to that (RP) ring, otherwise you'll start to get shunted off into Redstone 3 early builds, and those will be pretty unstable for a while. Just something to think about!

See also my summary of what's new going from Redstone to Redstone 2 (Anniversary Update to Creators Update).

Anyone with an existing phone that's officially deemed Windows 10 Mobile compatible by Microsoft can join in the fun. The full list of phones includes Lumia 430, 435, 532, 535, 540, 635 (1GB RAM version), 640, 640 XL, 730/735, 830, 930, 1520, Icon, BLU Win HD w510u, BLU Win HD LTE x150q, and the MCJ Madosma Q501. Plus, of course, all the devices which have shipped with Windows 10 Mobile in the last year, of which the Lumia 550, 650, 950, 950 XL, Acer Jade Primo and HP Elite X3 are the highest profile.

In each case:

1. Upgrade to Windows 10 Mobile (if appropriate) using the Upgrade Advisor.
2. After at least one update cycle you should then be on the Anniversary Update (a.k.a. Redstone). Head into Settings/Update & Security/Windows Insider Programme
3. Join the Insiders programme and choose the 'Fast' level in the pick list.
4. Within a few hours your phone should be seeing the build mentioned above and you'll be running next year's mobile OS today. Fun, fun, fun....!