Originally Posted by NickBaer

So I bought a new iPhone in January, and couldn't transfer the Google Authenticator data to the new phone. I had to start over with each site.

Sites that have implemented Google Authenticator don't have a good means to change from one phone to the next. I have had to ask them to disable, so I can re-enable.

Sites that do the SMS code, usually don't have a means to use multiple phone numbers. e.g. when I am in the US I have a +1 SIM card in my phone. When in the UK and EU, I have a +44 SIM card.