Seems a good idea, the only possible issue I can see is FMI uses cellular data as well as GPS to track the phone. So you need the phone to have constant 24/7 data connection, but naturally you probably won't want to put it on a monthly subscription contract just for that. I don't know if using a PAYG sim with a data allowance is a better idea, as 1) usually, if the sim isn't topped up it cancels, and 2) I don't know how much data FMI (or location services) will eat up, even if it's just in the background.