Use a 3rd party system such as Win911 or SCADAlarm. We developed our own internally that send messages via text, email, audible/PA and numeric pager. Users can acknowledge via web page, direct dial or any other method we want to implement 
This has a few advantages as well: If Lookout crashes/locks, it can send an alert that the main program has stopped responding. Using Lookout with DDe or OPC, you can also monitor the notification software if it fails. WatchDog for HMI and alerts.
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