Long-range 2-way remote

[xplod1236]

I need a remote to use with my diy car alarm. It has to be 2-way (data to and from the car), and a pretty long range (~.5 mile through buildings and other obstacles). There's not much data that will need to be sent, so transmission speed isn't a concern.

I've been doing a lot of thinking, and the best idea I came up with was using a cell phone on the user end, and some sort of internet connection (maybe another cell phone) in the car. I always have my cell phone (sprint, if it matters) with me, so I thought it would be nice to use that as a remote. My idea was to set up a webpage to which I would connect from the phone. Then, that data would be sent to the car through some internet connection. The range would be unlimited as long as I have service on the phone and an internet connection in the car. The only problem I see is that the phone might be slow loading up the web page. Another problem is the internet connection in the car. I could get a laptop card for internet access through a cell phone provider, but that would cost ~$50 a month, which is way too costly for this application. Free would be preferable.

Anybody have any suggestions, throw them out there. I'm open to any new ideas.