I’ve got Ubuntu Server and Latest version of Asterisk which is 11.5 installed on that.
during a day I’ve got problem with asterisk service, it seems that sometimes it freezes and stop working and some other times the service is gone and not even in top service lists.
is there any log or special configuration which i have to set?
You need a version built with no optimisation and with thread debugging enabled. Chances are that Ubuntu has neither of these set. Without them, it is unikely that any bug report will be progressed.
Google “asterisk wiki backtrace” for more information on getting information to debug crashes and deadlocks.