I used to work at the provider supporting these things. The newer (see at least 2 years old) CDMA cards I've seen don't support voice, this includes the Aircard 580 and ones after that. The older Aircard 555 does and it came with the software for your computer to dial up any number you wish.
To answer your second question, you can have voice and data on the same phone number, otherwise people wouldn't be able to use a Blackberry for email and phone calls.
But the best solution for you is to just get a $10 phone from one of your friends that they're not using anymore, plug it to your computer via USB, permanently wire a charger to it from the battery and throw it up under your seat somewhere. This way you have a modem that supports data and voice and the newer phones also have something called COPO or some ****, I just read about it today. It allows the phone to accept a voice call while in a data session.