Call his secretary and make an appointment with your attorney for a telephone conference or meeting. The purpose of the meeting is to establish a communication protocol, deadlines and address any issues that have already arisen because of the unreturned calls.
Consider some of these factors:
How much times passes before the calls are returned? What is the nature of the calls? Is your attorney limiting the calls to avoid unnecessary fees?
Are you organized and prepared with questions when you call?
How often do you call?
Do you leave messages with his secretary and indicate the urgency of the matters?