Update:
A new rumor claims that this Build 10.0.15234 will be the second preview build of Windows 10 for Phones set to be released to Windows Insiders tomorrow. You can have a look at some of the new features coming in the leaked video below in advance.
Time for a new Windows 10 for Phones build to leak in China. The build in question is the most advanced known build yet with Build number 12534 and you can watch it in action in the video below. You can see hands-on with the newly designed contact, messaging, Mail, Calendar and Spartan browser. Even the last leaked build 12531 had Spartan and new contact app. So it seems the Build that we may receive next week (hopefully) will come with these new apps and Spartan on-board.
If you want to read all the details and changes that Windows 10 for Phones brings no better place than our dedicated Windows 10 page.