Try a sample voicemail greeting like this one inspired by the 4-Hour Workweek.

Hi, you have reached the voicemail of John Smith with ABC Company. Please leave a message and I will get back to you as soon as I can. You can also leave your email address as I am often able to respond faster that way.  Thanks and have a great day.

This passes along the idea that email is your preferred method of communication. You can also go so far as leaving your email address and recommend that they send you an email INSTEAD of leaving a voicemail. But that might be a bit much.