Asterisk / Trixbox auto dial project.

We run a taxi company and currently have two modems connected to our computer system that rings the customers phone when the taxi arrives to pick them up. The taxi driver has a button in his cab which is linked to the office via his radio which in turn is connected to our computer system. When the button is pressed the computer dials the number via a normal modem and drops the call after two rings. I would like to get it so the call is routed through our asterisk telephone system and plays a recorded message then hangs up. Changing the method of how the call originates is not an option so the commands are going to have come directly from the Com port or from the PSTN side of the modem. Advice will be needed on any equipment we will need to purchase.