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.
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.