I'm using English for Telephoning at the moment, which is okay (not great). The publisher is Oxford Business English Series, by David Gordon Smith. I would say the difficulty is intermediate to upper intermediate. Their comprehension level must be quite good to understand the fast speech in the "phonecalls".
I'm working in a single company so I have the opportunity to print out emails sent from my students to native speakers, so we sometimes analyze the grammar used by their native speaking counterparts and try to produce better emails on their end. I'm also exploring the possibility of recording conference calls and playing them back for comprehension, cloze exercizes etc etc.