I think monitoring the event log is the easiest scenario. You
can pick your poison (Microsoft System Center Essentials, Nagios,
etc.) and have it send out an SMS alert when you come across Event
528's and 540's of logon type 10. Monitor 538's
(again of type 10) in order to see them log off.
Keep in mind that almost all major carriers have SMTP gateways
that allow you to send SMS messages using an e-mail address. For
instance if your number is 123-456-7890 and you're with AT&T,
you can receive SMS messages at [email protected] That way,
your event monitoring solution doesn't explicitly have to have SMS
capability. Do be aware that it's not perfect and can add quite a
delay in some cases. It's just a quick and dirty solution.