A couple of things come to mind. First off don't be too pissed that the phone is ringing, you can be pissed when it stops ringing

1. You might want to start conducting business like Richard Parr at PMO, give out a fax number only and get back to customers late in the day or early evening.
2. Do you have a girlfriend/wife/sister/brother that's looking for some fill in work?
3. As others have stated hire a secretary.
4. Try giving out your email address and suggesting to folks to use that to ask questions.
5. Unfortunately we are living in a society that expects instant info, and the most direct way is to call. Good luck