2FA is already a hassle for users. Now you want to make them do math too? This is not a solution. Just don't use SMS at all. Google Authenticator is a better solution than yours.
You make a good point about ease-of-use. I agree a phone app is much easier to use with a smartphone. However, people with flip phones couldn't install such an app. You might then argue the demographic with flip phones would either use an RSA device or not have 2FA enabled at all - which seems like a valid point.
Security-wise, having a secret user math function seems more secure than the Google app. I can give reasons why if needed.
Security-wise, having a secret user math function seems more secure than the Google app. I can give reasons why if needed.