Here is a quick video about effective voicemails from Jill Konrath: http://www.jillkonrath.com/sales-blog/bid/112301/Video-How-to-Leave-Effective-Voicemail-Messages-When-Prospecting
If you don't have a voicemail you are excited about, then why the heck are you leaving that message (i.e. impression, mental picture of yourself, undifferentiated introduction, lost-in-the-noice waste of time, etc.) ?
Overused insanity cliche: Insanity is doing the same thing over and over again and expecting different results. Expecting someone to call you back using a voicemail you are not excited about is insane. Expecting someone to call you back using your normal voicemail message -- when they never call you back -- is insane.
Regain your sanity. Take 30 minutes and develop a voicemail message that you would call yourself back about.