Speaking as a host, I´m quite happy for potential volunteers to telephone. That´s why we supply out phone number after all!
I also prefer direct emails to using the wwoof PM system, because it´s easier to reply via my telephone (which doesn´t seem to like the wwoof website!).
What irritates me hugely, however I´m contacted, is when people haven´t read the information that we´ve provided. There´s a huge amount of information on our website which is specifically for potential volunteers, and it would be great if people could read it rather than ask me the same questions over and over.