I'm hoping that those of you familiar with the industry can either let me know that the things I've learned (below in this email) are correct, or you can offer guidance to steer me in the right direction. I don't know the state of the industry today.

The best solution?
After some internetting, it seems that an extended Hayes-compatible command set sent through an RS232 port would satisfy this need. The modem would have to be an extended hayes-compatible hard modem.

The less-desirable directions?
I've found a USB Modem class from Mentor Graphics, as well as an interface called ACM, but neither of these seem to be supported much by the industry.

I also read that the other dominant Modem interface is US Robotics. I don't know how this interface is different than Extended Hayes, but it seems like it would tie me to one manufacturer and I don't know if that is a good idea for the next decade.

Lastly, I don't know how much of a "standard" that extended Hayes is or if there is a particular flavor that I should develop.

Nearly all modems supplied today will only operate with a PC with particular operating systems for which there are drivers (because the modems aren't anything without the software/driver). So you couldn't plug these modems into a different [embedded] device without appropriate driver [nearly impossible task to develop yourself]. Otherwise, you'd need a full hardware-controller based [expensive, almost obsolete] product.