Windows Phone 8 get mail over internet.

Hi

This regards Windows Phone 8 and Exchange 2013 running on Server 2012.

I can get an Outlook account set up on the phone in the office by using wifi and the local server name but I haven't got a clue how to do this over the internet.  If I put in the mail domain, user name, password etc. no link can be established.

Searches only tell me what I already know, something fundamental is being assumed!

Regards,

Andrew