Caledon Ken said:
I use the Google Authenticator app.

Syncs up with Gmail and Outlook.com, very easy to set up and no waiting for codes, they are on your phone by account.
Yes I agree that's a good way to go, as it means you aren't reliant on a phone service working.
I use the Microsoft version of the Authenticator app (as I have an old Windows phone) but it works almost exactly the same and is cross-compatible with Google, Facebook etc. as well as Microsoft.