Why is security number required for Hotmail?

I've had the same hotmail addy for nearly 10yrs. The only problems I've ever had occurred whenever Microsoft would make alterations or improvements, and they were minor. Unlike Microsoft, Google, Facebook etc., who readily fulfill request for information regarding their users, I'm not the type that is involved in social media, nor do I engage in other risky online behavior, which is why I've sailed pretty smoothly for so long, so - Why do I need to submit my personal information (phone number) for "security purposes"?