As I believe I said before regarding phone tracking of a car, if you don't make a call on a PAYG SIM or top it up for 6 months they may well disconnect it. Nothing is as easy as it seems.
Do you have to make a call on it, or can you simply just phone it? If the phone rings, I wonder if thats enough to count....? Because you could just phone it every so once it a while.
whats to stop you just sending a text every 6 months off it!? or even just topping it up £5 every six months! then, if you break down, you also have a phone to call the breakdown team on!