For a while, now the nerds at various Silicon Valley technology companies have been asking for you to hand over your phone number for "security reasons". There is no reason for this to happen of course, and it does put you in the exposed position of not knowing where that number is going to be used. Even if Microsoft does not use the number to cold call you at all hours for "must have deals" like those idiots at Vodafone, there is no guarantee that your phone number will not fall into the wrong hands.
Now Windows 10 Build 17063 demands that you give your mobile phone number to Microsoft during the setup process. If you refuse to give the number, you will not be allowed to finish installing the new build on your PC. The screen that demands your number says" "Link your Phone and PC. Windows loves all your devices-get essential apps that help you work better across your devices."
There is no "Skip" button, and when you enter your mobile number, an SMS message is sent to your smartphone that links the two devices. This allows you to use features like "Continue on PC" which is an app that allows you to start looking over a website on your phone and finish it on your PC screen.
We have been here before. Earlier versions allowed Windows 10 users to skip this action if you did not want it. It is possible that Vole has installed a skip button that users just can't see, but we cant be sure. This is strictly an insider build so if there is enough of an outcry the button might reappear.