From the Cacti web site:
One user has developed plugin to monitor and graph Asterisk.
Update 2007
We at ITConnection.ru are happy to post a Cacti script for Asterisk IP-PBX statistics. It’s based on Python, works over Asterisk manager API connection and provides cute and comprehensive graphs. You can download Cacti’s plug-in from this page: http://forums.cacti.net/viewtopic.php?p=111317
PS: asx-stats-0.2.0.tgz too many bug to use! And asx-stats-0.3.0.tar.bz2 link was dead!