There has been previous discussion on using MagicJack for with X10's security system. I don't have a security system installed, I use MagicJack extensively in my home-based business.
My personal feelings, based on my MagicJack experience and many years of doing telephony tech support, is that if you consider your X10 security system a critical application, MagicJack is not reliable enough.
First there is the power situation. If your home loses power and you don't have battery backup for your X10 pc and your complete DSL/cable system end-to-end, your security system will not be able to dial out. If your batteries exhaust before power is restored, you will lose the outdial capability.
An old-fashioned landline type of phone does not have this problem as power is supplied by the phone company.
Secondly, the MagicJack pc must be running for it to dial out.
Third, I find that MagicJack is not reliable enough for a critical application. Sometimes when I make a call I get such noise on the line that I have to disconnect and dial again. Usually I get through on the second attempt. And I have trouble reaching certain area codes. This may or may not be a MagicJack problem.
But, there are folks on this forum who use Magicjack for their security system and I'm sure they will respond also.