If you install Asterisk on Ubuntu via apt-get, and then remove it and re-install again, you may find that you are missing the all the config files in /etc/asterisk.
This is a bug in the debian asterisk package that means that the config files are deleted when you remove the asterisk package even though they are part of the asterisk-config package.
To fix it simply run the following commands: (NB: This will nuke any asterisk installation you have)
sudo dpkg --purge asterisk
sudo dpkg --purge asterisk-config
sudo apt-get install asterisk-config
sudo apt-get install asterisk