Grandstream 2000 phone can't dial another phone

I have a grandstream 2000 phone with trixbox 2.6?. When I try to dial an extension there is a voice that comes on and says invalid extension. When I pick up the handset, the line says unregistered. what do I need to check?

In the PBX section under modules, I had not loaded all the module. The basis unit came unloaded without most modules.

There are two things required for your phones to be able to call each other. (1) go into the phones HTML setup screen and provide the server IP information, the user/login extension and the password. Also provide the proper gateway so the phone will display the proper time/date.
(2) go into the extensions area and add that phone. Normally a Generic SIP Device and be sure to put the proper extension and passwords in all areas.
If the phone says unregistered then it is not connecting to your server. If it doesn’t show the proper date/time then it’s not making it outside to the internet thus not set up properly-IP/Subnet/Gateway. This is assuming the default time setting. Hope this helps.