how do I change phone number to receive verification code?
Phone number listed when I set up keychain cannot be accessed, how do I change it to receive verification code?
iPhone 4, iOS 7.1.2
You can only change the number once you are successfully in the Keychain. Get help using iCloud Keychain - Apple Support
In your case, if you cannot receive the verification code and you do not have another device already logged into iCloud Keychain that you can approve from, then you will need to choose the option that says forgot code. This will allow you to reset the keychain.