I would not recommend using magicJack for an Alarm System that is monitored by an Alarm Company.
The Alarm Company needs to have a dedicated phone line for direct communications 24/7. That phone line should not be shared for voice calls because the alarm panel WILL cut you off to make data communications whenever it needs to without regard to how important your phone call was. The newer Alarm Systems now have cellphone technology to avoid wire cutting attacks but are also more expensive to monitor.
You have to decide what is more important; your life & property OR trying to save a few bucks on telephone costs in monitoring. (Besides, most alarm panels will protest when their telephone line has lost a connection and that will happen whenever you shut down the computer hosting the magicJack).
PS: This is also true for elevator cars (all elevators have an emergency call "box"). Don't even think about using a magicJack for the elevator systems.