Yesterday, Microsoft released Windows 10 for PCs Insider Preview build 16273 to the Fast ring, bringing a range of new features that will debut in the Fall Creators Update. Despite there being no new Windows 10 Mobile builds, it would seem that the company also pushed Mobile build 15240 to the Slow ring yesterday, despite not giving any indication of its release.
Indeed, Windows 10 Mobile almost never gets a mention anymore when a build gets pushed to the Slow ring. The original blog post for build 15240 hasn't even been updated, as a PC build post would have been. In fact, when this build was first made available to the Fast ring, it was on a day when Windows Insider Program chief Dona Sarkar had said that there would be no new builds.
Windows 10 Mobile build 15240 comes from the feature2 branch, rather than Redstone 3, which is where PC builds come from. This build actually triples the amount of new features coming in the update, to a total of three. The two new features included are support for Emoji 5.0, and support for the Chinese Lunar Calendar.
Earlier this week, the company also released Mobile Emulator Insider Preview build 15240, alongside SDK Preview build 16267. If you're a developer, these might be worth checking out.
If you're on the Slow ring, you can update your phone to the new build by going to Settings -> Update & security -> Windows Update -> Check for updates.