I'm out of my element here

Right now I'm supposed to be talking on the phone to vendors, just to verify some information and collect information that we don't have. The problem is, I'm terrified of speaking on the phone to people I don't know, mainly because I barely grasp the business I work for. I just don't know that much about accounts payable, and I'm supposed to sound like I know what I'm talking about when I call these vendors. Some of these vendors are individuals who run their own businesses, who have maybe been used once by our company, yet we need to go through the whole process of setting them up as a vendor (aka red tape). I understand it's necessary, but I've already talked to a couple of them, and they either don't understand the reason for the red tape ("What's a W-9 form, and why do I need to send you a copy?") or they think it's ridiculous that I'm calling to ask them for information which we already have. I hate giving the impression that the left hand isn't talking to the right hand around here, because we're a pretty big, well-established company, and they've designated me, a temp who doesn't know much in the first place, as the liaison between said vendor and said large company. Also, there are a lot of more qualified individuals within earshot of my cubicle, who undoubetly have overheard me make a fool of myself while talking to these vendors. Like I need that extra pressure.

So I'm freaked out. Freaked out hard.