Go to your PBX interface: pbx.mixpbx.net. Enter your logins : username and password.
Access the Extension section. Add a new extension.
Number Enter the number you want to assign to the phone (except 112 (ambulance) and 113 (police)). Mac Enter the Mac address that's written on the back of the phone (begins with 00085Dxxxxxx). Apply and do not forget to PUSH the configuration.
Acces the Webinterface of your Phone by entering the IP address in your Browser (how to get the IP address). The default logins are username: admin password: 22222 (if the phone is not yet configured).
Click on Configuration server (1).
Download Protocol select HTTP (2).
HTTP Server type in your registrar (3) (username.mixpbx.net).
HTTP Path type aastra.php (4) and Save the settings.
Click on Reset (1), then on Restart (2) and confirm that you want to restart the phone.