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Support Received "Account action required" notification with unrecognized number

I received a notification on my phone saying I had changed my number and asking me to verify it but I did not recognize the number. I couldn't do anything about it at the time but a few hours later I changed my password.

To receive this notification someone would have had to have access to my account, right? Is there a way they can try to change phone number if they just know my email. If they did have access to my account, it doesn't look like they made any attempt to change my password or use my email to change passwords for other sites.

Is there somewhere I can report the number?
 

Related:

#1 Shifty on Android, Sep 9, 2017