Pop up new iPhone signed into my account and I do not see another device

Open Settings ➔ Apple ID (Your Name at Top): Here, at the bottom, you will see all the devices that are signed on to your Apple ID. If you do not recognize a device or find a device that shouldn't be there then change the password to your Apple ID immediately. If all the devices appear to be correct then there have been no unauthorized sign ins.

It is nearly impossible to hack into an iPhone due to Apple's closed environment. Apple ID and Password get compromised through phishing scams and emails.

Axel F.

Accountant15 Said:

"Pop up new iPhone signed into my account and I do not see another device: Pop up came up on my iPad and iMac that another iPhone signed into my account does this mean I have been hacked?"

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Is this a Text Message?

Apple would not send you such a text message out of nowhere. So, if nothing else is seen, it seems to be a scam. Take a screenshot, and forward it to Apple. Go Here: Using the Photos App to Report Scams - User Tips