In a phone conversation you are having an actual conversation. You can hear a human, the tone of the voice, hear if the other person responds immideately or hesitates. You get lots of contextual information you can't get from text alone.

And as the name implies a telephone conversation is an actual conversation. A conversation involves interaction.

With twitter you are not so much in a conversation as you yelling and hoping that gits on the internet will give you attention.

So yes, I'd say there is a difference and it lies in how the medium works, not in its implementation (i.e. digital or analog).