Working with IT can sometimes be a pain. You do everything by the book, but it is still not working. (if I got a penny every time i ran in to that… 🙂 )
Enabling External Communiations/Federation in Lync Online is one of the easiest things to do, enable a tick box and make sure that you have all dns records added for Lync Online to work properly.
What if its not working anyway? Recently that was exactly my case, I had done the extremely difficult operations above, yet external federation was not working.
After troubleshooting forth and back by enabling logging in the Lync clients, it seemed like my external communiations settings had not been provisioned to back end. The solution I tried then was the following:
Simply disable external communications in the portal and wait 24 hours before you enable it again.
That did the trick! Sometimes there are easy ways to solve problems. 🙂