tell application "Finder" to set visible of process "magicJack" to false
Save it to your /Library/Scripts folder. Next, create a cron entry (mine is set to run every five minutes) to run the script:
*/5 * * * * /usr/bin/osascript /Library/Scripts/minimize_magicjack.scpt
[robg adds: I haven't tested this one. cron isn't the officially-supported OS X solution, but it still works. This ancient hint contains some basic information on how to use it.]

