I have a magicjack and although it has improved quite a bit in the relm of reliablity I would say using it for monitoring would be a really bad idea. It also like all VOIP setups isn't designed to relay data it's designed to relay the human voice only so just getting it to work at all would probably be an effort.
In the past my magicjack has had serious connectivity and quality issues, pretty much what you should expect for $20/yr phone service. However in the last couple few months it has been pretty reliable and I do use it for remote control.
You might look at one of the many services that are designed to do this as they aren't that expensive and have a crapload of research put into their reliablity.
Now if this was self monitored, that might be different but when you use the duress code you dan sure want that to work.