Problem with instace

Hy, i’m using centos and final release of astzerisk, but i have a small problem, sometimes my instance of proccess goes downa and it is zero.

I want to add in cron job that every 15 minutes check my asterisk and if asterisk is down automaticaly it goes up- But i don’t know how to make that, i’m begginer in linux and i cant find which file i must input in my cron file.

Please help.