Yeah, this is what I always figured. When you're face to face with someone and experiencing a block of some sort, they have visual cues and realize this. They wait. When I'm on the phone, the other person wonders if I've hung up. "Hello? Still there?" If I have a block while confirming I'm still present, then it's not unusual for them to just hang up. It really does create a difference sense of urgency when the only cues you have to give are verbal. The more times you experience the hang-up situation, the more you realize that phone conversations are unproductive and rather stressful. Likewise, I'm sure a lot of stutterers have that one terrible phone call experience that haunts them and exacerbates the anxiety. Mine was having an angry boss force me to call a customer while standing right beside me to make sure I didn't screw it up.