It's the start of a whole new cycle on Windows 10 Mobile, at least for Insiders on the 'fast' ring, with the very first of the 'Redstone 2' branch builds now available, build 14905.1000. This branch isn't due for public release until Spring 2017, but you can get a head start by upgrading now and then following Microsoft's progress through August-March. There's little new on the surface yet, mind you, aside from new system sound effects, with the main changes being under the hood, with a new version of 'OneCore' the core code that underpins all modern versions of Windows.

Following on from yesterday's release of Windows 10 Mobile 14393.67 (i.e. Redstone) to all production devices, here's the new Microsoft blog post:

Here’s what’s new in Build 14905 for Mobile

• We are introducing a newly refined sound set in this build, uniting the best of our past and present. We aspire to set a new bar for mobile sound set quality, and are trying to make the soundscape of technology more beautiful and harmonious. This also helps align with new sound design direction of the Windows platform as a whole, so that mobile sounds will family with desktop and tablet and be feel instantly familiar to all Windows users. We will continue to evolve – head to Settings > Personalization > Sounds to see the updated list of available sounds and let us know what you think!

Improvements and fixes for Mobile

• Missed call notifications are now more actionable, with inline options to call back, text or remind yourself to do something about it later.
• We fixed an issue where videos played in Windows Phone 8 apps might not pause when an incoming call was received.
• We fixed an issue where, if “Show my caller ID” is set to “My contacts”, the contact being called might still see a blocked caller ID.
• We fixed an issue where the Lock screen might fail to update to the new time after a time zone change.
• We fixed an issue resulting in music not resuming after a call was finished, if Turn-by-Turn directions were being read out from the Maps app when the phone call came in.

Known issues for Mobile

• If you move apps between a SD card and internal storage (either direction), those apps will get stuck in a pending state. The workaround to get your apps working again is to uninstall the app through Settings > System > Storage (apps cannot be uninstalled from All apps list). Then you can re-install the app from the Store.
• Cortana’s text to speech capabilities is not working in this build. For example, Cortana won’t be able to read text messages out loud for you, tell jokes, sing, or give verbal prompts. 

One of my devices is  downloading and updating as I write this. Curiously, it's my hacked Lumia 1020 - go figure! Might have been because I was pretending the 1020 was a 950 Xl.... [ahem] So at least the 950 XL should already have it. Data points welcome on devices and regions, as always, until the rollout is more complete! I'll update the post later if there's anything specific to share.

After the Redstone 2 Insiders update (first build)

Some of these sounds do seem new, more investigation needed!

Of course, in the fulness of time we'll need a summary of what's new going from Redstone to Redstone 2, but we won't have a full feature set or changelog until at least Christmas 2016. Settle down for the long haul, everyone....!