To use Apple’s two-factor authentication (2FA) with your Apple ID account–something that the company has effectively mandated for the last few years–you must have at least one “trusted phone number” associated with your account. A trusted phone number can receive a code via SMS or an automated voice call speaking the code to validate a login on a new device. This contrasts with a trusted device, which is any iPhone, iPad, or Mac logged into that Apple ID account.

You can add more than one trusted phone number, which is useful for backup in case a phone is lost or a phone number is hijacked. But you must have at least one.

Some readers find that they can’t set up their Apple ID for 2FA with a number already associated with another Apple ID account, or they can’t add such a number as a second or subsequent trusted phone number. They receive an alert that the number is already in use by another Apple ID account. This is particularly irritating for many people who rely on verifying their Apple ID login using a trusted device and never rely on the trusted phone number at all.

Apple hasn’t documented this limitation. To me, it seemed like a change, but I can find people posting about receiving an alert like this dating back a few years. I just tested adding to my list of trusted phone numbers a family member’s cell (with their permission) that’s already assigned to their Apple ID. I had no alert or difficulty.

One reader found this particularly vexing: they had an old Apple ID account they no longer use, but because their current and only phone number was associated with that account, they couldn’t finish setting up a new account. They were looking into deleting the old account by calling Apple Support; I wouldn’t just try to delete it, as there’s no knowing whether deleting the account will truly free up the old number.

Other posters on the internet have found they were told the number was already in use with an Apple ID–but not by them. They had new phone numbers that the previous owner had clearly connected to an Apple ID account and never disconnected or deleted.

Beyond going to support.apple.com to set up a tech support call with Apple to try to resolve the situation, you could just acquire another phone number. Google still lets you set up a Google Voice account at no cost and assigns it a legitimate phone number. You can then add that phone number as a trusted one. You can configure your Google Voice number to forward text messages to email or access the account directly via a web browser or an iOS app.

This Mac 911 article is in response to a question submitted by Macworld reader Zane.

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