My recomendation: I hear boost mobile's i415 will function as a 19000 baud modem over usb without using any credit on the phone, the phone is cheap, and you only need to credit the phone every 3 months or so $20. Get that.
It is a heck of alot easier making a auto connect to the internet script than trying to figure out how to interface and automate text messaging to a different phone. And to make all that obsolete, the phone has a GPS built in (nextel based) so you wouldn't need your own, and there is a third party app for transmitting with it. Check it:
MAKE: Blog: DIY GPS tracking with Mologogo - review
It notes that web costs .20 cents a day, but maintaining live service means crediting about .22 cents a day to the phone anyway, so its like the service is free if you keep the phone alive.
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