Managing personal Outlook.com account settings, security, and privacy
Hi!
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According to your information, there is already an attack on your account (if you verified another number showing on sign in method)check as well if this is an old number that you own. You have to take immediate action and verify your account through the Microsoft Security page: https://account.microsoft.com/security
Once logged in check for anyecent activity (look for unfamiliar locations/devices), security information like a unfamiliar phone number.
Once you have validated the details, Immediately change your password (from a safe device using your own home internet) use a strong, unique password( don’t reuse an old one). Next, remove unknown security information, go to: Security → Advanced security options and check phone numbers or recovery addresses, REMOVE anything that you don't recognize.
For extra information, check “Recent activity” and you might see the following:
IP location (approximate)
Device/browser
Success/failed attempts