The first phone call is a big thing for me. Before I tried online dating, I never realized how much a guy's voice mattered. I'm usually more of a visual person than an auditory person, but a guy's voice is a big factor in determining if I'm attracted to him or not. When you meet someone (you know, in the normal way), you meet the entire package... looks, mannerisms, behavior, voice are all bound into one identity. It's hard to separate components of someone's personality. The only time a voice is really noticeable by itself is if there is a huge discrepancy between looks and voice. Like if a big burly guy has a high, squeaky voice, or if a buxom blonde has a tinny, whining voice (a la The Man With Two Brains).
But when you meet someone online, you don't get the whole picture all at once. You get a picture, a few if you're lucky, and a few paragraphs about him, or at least how he has chosen to portray himself. It tells you some superficial information about him, maybe a little something deeper if he's a good writer (or if you're a good analyzer), but you won't get a real idea of who he is until the two of you speak. Of course, the first face-to-face meeting will tell you much more, but you will at least get a sense of his personality and larger identity.
There was one guy awhile back (one of my first online dating experiences) whose voice ruined it for me. He seemed really nice, and he was cute. We had great email conversations, and I was pretty excited to meet him. The first phone call finally happened, and when I heard his voice, I almost sighed in disappointment. It just completely turned me off. Still, I gave it a shot and met him, but it was to no avail. My emotional disconnect with his voice was mirrored in my feelings about his personality, too. The voice just says so much about personality.