Asterisk administrators set up call routing using the Asterisk dialplan, which resides in the /etc/asterisk/extensions.conf file. The dialplan is made up of a series of call rules called extensions. When a phone call comes in to the system, the dialed number is used to find the extension in the dialplan that should be used for processing the call. The extension includes a list of dialplan applications which will be executed on the channel. The applications available for execution in the dialplan are maintained in an application registry. This registry is populated at runtime as modules are loaded.