Input yourextension@yourdomain in the Username field.
Input the secret of the extension into the Password field.
Click "Create an Account". (You can copy and paste the secret if you are willing to e-mail it to yourself and you have access to your e-mail via your phone.)
Input the domain of your phone system into the "Hostname or Provider" field, then click "Next" (replace customer with your domain name).
Skip the step for Authentication Username and Outbound Proxy.
Select "SIP UDP" for the phone configurations, then click "Finish". (SIP UDP will show as found when you connect to the internet--if you configured correctly.)
Configuration for iOS:
Go to the Settings tab.