Error when registering

I’m totally new with VOIP and SIP, so please be patient! I have a Freepbx box set up, and am using Csipsimple on my Android to use as an extension phone. The problem is that I get the error message “error when registering - not implimented” on the phone. How can I trouble shoot this?

It did register for a while, but no longer does.

Thanks for the help.

James