Good morning all,

We're currently in the process of migrating our Exchange server from 2007 to 2013, and am experiencing a few problems. At the moment, OWA redirection works fine, but ActiveSync does not. At the moment the 2013 Server isn't public facing, it's internal until all testing is complete. However despite being on the LAN, when an iPhone is configured to target the 2013 server, it isn't proxying to the 2007 CAS, but instead provides a vague error of being Unable to Verify account information. No mail is retrieved.

Has there been a step I've missed to allow this to work seamlessly? 

See this article: http://technet.microsoft.com/en-us/library/hh529912(v=exchg.150).aspx, and make sure that you did all steps listed here.