I had a strange issue today, one of my phones couldn't be dialed by other phone but it could dial out without any issues.
When someone dialed the extension they just got a busy tone.
After trying a lot of different things I went for help on IRC in the #freepbx channel.
Fortunately I got the advice to rebuild AstDB and that solved al issues I had.
Rebuilding AstDB is much easier than it sounds, just follow these steps:
1. Stop the asterisk service (in FreePBX you can just type: amportal stop)
2. Change your directory to: /var/lib/asterisk/
3. Rename astdb to astdbOLD (just type: mv astdb astdbOLD)
4. Start the Asterisk service again (in FreePBX type: amportal start)
5. In FreePBX go to extensions, click every extension and click submit on the bottom of the page.
6. When finished doing this click: Apply Configuration Changes