There is a review of this in the XDA forum thread:
How to talk to the Modem with AT commands.
That thread show you how to send AT commands (ATC) with a remote terminal (USB connected to you PC), but it has not yet solved the problem of how to use a local (phone) terminal to talk to the phone modem. This is dependent on both the modem chip set used in your phone and how the phone is set up when connected to a USB port. There are usually some kind of engineering menu (AKA secret code) that you can dial to get the phone to act like a modem or PDA.
BTW. You need a terminal application/program to do any talking to the modem (remember, it's a two-way communication). So that's why you need a rooted device, since the rootkit usually come with BusyBox (that includes the
microcom terminal program). Otherwise, you have to write your own program.