Of course sales of Windows phones has fallen...windows 10 mobile is not only in beta testing. People are wating for the operating system to be finalized before buying a phone. In addition, there are a very limited number of phones running windows mobile at all.
Once more are released the numbers will skyrocket! I would love to get a Sony phone running Windows 10 mobile if they would bother to release it in the US!
Watch out iPhone...why you think Apple is rushing to release cheap phones worldwide...they know perfectly that Windows 10 mobile is going to explode once they release the final version of the OS and they have phones out there to run it from Asus, Sony, and of course, Microsoft!