warning message
I received a message stating that my Apple ID and phone number are now being used for iMessage on a new iPhone. I want to check the security of my account, please.
iPhone 7, iOS 13
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Receiving those sorts of messages is common. If you added a new device or signed into iMessage in, say, the last 24 hours you might be getting a delayed message. I assume you chose "OK" to not challenge the message.
Regardless, assuming you are using two-factor authentication, Check your Apple ID device list (https://support.apple.com/en-us/HT205064) and remove any device that shouldn't be signed in using the steps in the link.
If there are none that shouldn’t be signed in, then you should not have a problem at this point. But, if an unauthorized device signed in, Change your Apple ID password https://support.apple.com/en-us/HT201355
Here is What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071
And see these security-related Apple Documents:
If you think your Apple ID has been compromised https://support.apple.com/en-us/HT201303
Security and your Apple ID https://support.apple.com/en-us/HT201303
iCloud security overview https://support.apple.com/en-us/HT202303
Ways to keep your information safe on Mac https://support.apple.com/en-gb/guide/mac-help/mh11402/mac
Using app-specific passwords https://support.apple.com/en-us/HT204397