This is a basic extensions.conf (the Asterisk dialplan) configuration for Asterisk v1.8. As you can see, only two lines are actually necessary in extensions.conf for the SIP user agents defined in sip.conf to be able to call one another.

If you’re confused, than examine this dialplan in conjunction with my previous blog post on configuring a basic sip.conf in Asterisk 1.8. It may also enhance your understanding of Asterisk to examine the script I posted that complies any version of Asterisk in Ubutnu.

The default location for extension.conf on an Ubuntu 10.04 and 12.04 system is:
/etc/asterisk/extensions.conf
The configuration file is commented for maximum clarity. The active code […]