Effectively using voicemail?

Recently my texting game has been 10x better than before. Thanks to the Manwhore for the guide. But I really feel like some great voicemails would really solifidy myself as a top contender for most of these girls. However I generally feel like I don't have much to say unless I'm specifically calling for a reason, and I even think some girls are embarassed to talk on the phone to a guy they may have just met or like for the first time. What are some tips on what kind of messages I should be leaving? Maybe something along the lines of accusing them of a crazy reason for not picking up...

Open to suggestions!