I hate the telephone (despite spending most of my career, thus far, at Ma Bell or one of her progeny). Absolutely, unquestionably, objectively, the worst thing invented, ever. So, to answer your question: if I want to strike up a conversation with a Twittering friend, I send him a response on Twitter, or an email, or IM/chat message. But I only have three friends and only one of them uses Twitter, and I suspect he does so in write-only mode.
Actually I don't post much original material to Twitter ;-) -- most of it is auto-posted links to my blog posts.
--ravi