GPS Relay Capability?

My friend's company provides cell phones to their truck drivers. He
would like to track their location without adding new equipment to the
truck. I thought the built-in GPS capability of newer phones might work
for this. I'm envisioning an application that we can run on the phone
which will send a text message containing the GPS coordinates every 30
minutes. Is anyone aware of phone models which will run custom software
and provide access to the GPS coordinates? Or perhaps a phone which
already has such a feature built-in?

Thanks for the input.