I can gauge emotion on the phone. I can hear inflection. I have a much better feel for the person than i do on IM.
There is a focus necessary there. You are talking to me. You may put me on hold for a few minutes to talk to someone who just stopped by, but you are talking to me. I am your focus. You can’t just ignore me or forget about me or get distracted without letting me know like you can when you’re IMing. Plus i’m focused on you. Online i may be doing other things or chatting with other people when you IM me. The one-on-one focus goes both ways. Yes, i am an attention whore; i want it to be all about me.
It is much easier for me to babble verbally than when i have to type it all out. I write long-winded livejournal entries all the time; i am tired of typing out everything i do or think. So i am much more likely to talk, period, whereas on IM it’s so easy to be non-talkative.