Microsoft made available to at least some Fast Ring Windows Insiders an updated build of Windows 10 Mobile -- its first new test build released in 2016.

The new build, 10586.63, began trickling out to some with Windows Phones at the end of the day (ET) on January 8. The new build is said to be a Cumulative Update. As of 4:30 p.m. ET, Microsoft officials had not released any information about what kinds of features or fixes were in the build.

In early December 2015, Microsoft delivered its first Cumulative Update for Windows 10 Mobile to Insider testers. That build was 10586.29. Days after pushing that build -- which included primarily performance and stability improvements -- Microsoft also released that same build to Windows 10 Phone users who were not running Insider preview builds. Windows 10 PC and tablet users also got a 10586.29 release at the same time.

Microsoft subsequently released its second Windows 10 Mobile Cumulative Update, 10586.36, a few days later to its Insider testers.

I've asked Microsoft what's new in today's Windows 10 Mobile build. No word back so far. Windows Insider chief Gabe Aul tweeted a few minutes ago that he would be communicating more about what is happening with the new build.

Update: It seems there was a last-minute problem detected with the staging, resulting in Microsoft temporarily pausing availability of the update, Aul tweeted. The company (as of 4:50 pm ET) is resuming the rollout of the build to Insiders in the Fast Ring today, he said.

Update No. 2: The "official" explanation of what happened on Friday re: 105862.63 is here. There are no specifics from Microsoft on that page about what's included in the new build.

Update No. 3: The unofficial word as to what's in 105862.63 looks to be on Reddit. I don't know if this is real, but it looks plausible. (Thanks @MobileTechWorld.)

Aul also tweeted that there are no new features in this test build. "Mostly upgrade fixes and corner-case things per device model," he said when asked for a list of what's new.

Today's Windows 10 Mobile build isn't the one that Microsoft, other Windows Phone makers and carriers are planning to make available to Windows Phone 8.1 users. Microsoft officials said at the end of last year that the upgrade to Windows 10 Mobile would begin rolling out to some subset of existing Windows Phone users in early 2016.