sorry but I am in France, I don't know where you live (and don't know for other countries actually).
It's actually 1.5€/month, and you can leave when you want!!
Even after one month.
And it counts each second (not each whole minute!!).
And no minimu of call per month... EVen if you never call, you do not loose the subscription.
Here is the link in french:
http://www.prixtel.com/particulier/offre-telephone-mobile/forfait-chrono-sans-engagement/chronoOf course the cost per minute is a little bit more expensive than other: 0,12€/minute
But who cares, it's just to send an alarm, so it does not happen that often...
BTW I did have already one like this for my car, so If I have an accident, it calls the police.
I will subscribe to another one for the alarm purpose
I hope you have the same in your country
Rod