I've heard of a few 3G dongles that have a bluetooth audio option. It's not really the same as the Pandora becoming a phone, all the Pandora would do would be to send the right AT command to dial a number and you'd need a bluetooth headset connected to the modem for audio, but it is the closest I can think of right now.
There are 3G modules, like chips and stuff, which act as a usb composite device of both modem and sound card, if you were feeling adventurous you could turn one of these into an actual USB device and embed it somewhere in the Pandora, wired directly to the USB port. This is the current plan for the Pyra, as far as I understand it. You'd need some software to go with it, but it's all just AT commands and audio ports, something that should be pretty simple if you've got the technical knowledge to build a USB device almost from scratch.
So can the Pandora be a phone? Yes, with some caveats and a lot of hacking. Otherwise VOIP is your only option.