I'll give you a quick rundown including links to the docs. You're going to have to make a new "time period". read this:
nagios.sourceforge.net/docs/2_0/ ... timeperiod
and then you're going to have to make a new contact (sms-user perhaps?) in your contacts.cfg file. read this:
nagios.sourceforge.net/docs/2_0/ ... ml#contact
Read each entry there and i think you'll get the gist of what criteria your new contact will have. basically, set appropriate host and service notification periods (the timeperiod you just made), set the service and host notification commands to 'notify-by-epager' and 'host-notify-by-epager' respectively (if that's the defined command you're using, you mentioned already being able to SMS), set the "service_notification_options" and host_notification_options to "u,c,r" , then finally add your new sms user to whatever contactgroup you are using in your service definitions (edit your contactgroups.cfg).