How can I configure icinga/nagios to alert via sms?

Using a 3rd-party SMS service, or (even better) using a GSM device as Craig suggests, is going to be much more reliable/robust.

If you're dead set on using the provider's email->SMS gateway, you can use slimmed-down contact commands like this:

define command {
    command_name    notify-host-by-sms
    command_line    /usr/bin/printf "%b" "$NOTIFICATIONTYPE$\n$HOSTNAME$ is $HOSTSTATE$\n$HOSTOUTPUT$\n" | /usr/bin/mail -s "[Nagios] ($HOSTNAME$ $HOSTSTATE$)" $CONTACTEMAIL$

}

define command {
    command_name    notify-service-by-sms
    command_line    /usr/bin/printf "%b" "$NOTIFICATIONTYPE$\n$SERVICEDESC$ ($HOSTNAME$) is $SERVICESTATE$\n$SERVICEOUTPUT$" | /usr/bin/mail -s "[Nagios] ($HOSTNAME$/$SERVICEDESC$)" $CONTACTEMAIL$

If any of these end up being too long, you can remove or substitute different macros in several places, as necessary. See the macro list for info on what's available in a notification command.

In contacts_nagios2.cfg:

define contactgroup{
        contactgroup_name       admins
        alias                   Nagios Administrators
        members                 user
}
define contact{
        contact_name                    name
        alias                           name
        service_notification_period     24x7
        host_notification_period        24x7
        service_notification_options    w,u,c,r
        host_notification_options       d,r
        service_notification_commands   notify-by-sms
        host_notification_commands      host-notify-by-sms
        email                           mail
        pager                          MOBILENUMBER
}

And in commands.cfg

define command{
       command_name notify-by-sms
       command_line script-to-sms-service
}
define command{
       command_name host-notify-by-sms
       command_line script-to-sms-service
}