I have this iPhone 6.
Even though Steam instantly sends the Steam Guard code email, you may encounter a delay with some email providers depending on their server load and processing times.
If you still have the code, go here to remove the authenticator.
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If you didn't write down your recovery code, then you will need to contact Steam Support.
Then 2 years ago I changed my number as I moved to a new state and didnt want to deal with the hassel of giving out an out of state number.