GSM Home Security Systems--What do you know about them?

Ok. So I bought one of those GSM security systems. For those who may just be tuning in for info themselves, these are those unmonitored burglar alarm systems that, in case of a breach, will dial several numbers that you would have programmed into the system. The GSMs take a sim card.

What I need to know is, how does this work?--

1--Is there such a thing as an ‘unlocked’ sim and I can use any, or would I have to already have a carrier, and get a sim from them?

2--Will it automatically work on insertion?

3--Will I have to pay a separate phone bill?????

I didn’t think of these things until now that I am installing. I had assumed (& yes. I know what assumed means :) that

#1—yes as far as getting a sim from my carrier. That is only logical.

#2—yes. Because the phone companies can’t do anything with security systems, and

#3—there would be no separate bill because if you’ve got to pay another phone bill, where is the savings all these systems claim???

*And YES, I’ve called my carrier --and another-- being I will switch to the one that can guarantee me what I want, but they haven’t a clue.

*And the seller of the system is just that, a seller of various merchandise and can only tell me what they read on the box.

I've also searched old answers here to no avail.

What I am hoping for is answers from personal users/installers.

And what I’m -really- hoping for is no trolls. (:

But if you’ve got anything constructive beyond--set it up, spend money on a sim and hope it works…and for free, I’ve already arrived at that conclusion a while back. :)

But I will still 'listen'.

Help please. And thanks.