Never do the ol' "Testing, check, check" thing verbally into it, using your own voice, for obvious reasons. Use "tongue clicks" instead.
Also, the cellphone app "MapQuest FindMe" is awesome. About $5/mo., as I recall, and you can set it up to send you an e-mail whenever the phone comes within a predetermined distance (say, 100 yards) of a predetermined location (say, OP's house), spouse's atty's office, etc. Same deal applies here -- disable ALL of the sounds on the prepaid cellphone, and put a small piece of black electrical tape over the speaker just in case. Hide it in the trunk of their car; if you either own or co-own the car, it's perfectly legal for you to be informed where your own property is located.
