Microsoft is making a new Windows 10 Mobile build, No. 10549, available to Fast Ring Insider testers.
The new build includes a bunch of bug fixes, along with improvements to Cortana and the Messaging app.
In order to get the new build, users need to go back to Windows Phone 8.1 first using the Windows Device Recovery Tool. Microsoft decided to go ahead and push out this build with this requirement rather than making testers wait longer to get an updated Windows 10 Mobile preview release. (Testers also can just opt to stay on Build 10536 if they don't want to go back to Windows Phone 8.1 first.)
Cortana now works for Insiders in Japan and in English for testers in Australia and Canada, as of this new build. The Messaging app now has a text box that can be expanded as users enter text. This build doesn't yet include the integrated Skype messaging, phone and video apps that are part of the latest Windows 10 test build (10656) for PCs, however.
Microsoft fixed a number of issues with this build, including bugs involving the Windows Camera app, the touch keyboard, call-blocking and screen-shotting with certain apps.
For the full list of what's fixed and known issues, check out the October 14 blog post about the newest Windows 10 Mobile build.