Voicemail drops leave a voicemail (that you record) on a mobile phone without ringing the phone. It will show as a missed call and leave your voicemail. The prospect then listens to the voicemail or sees the missed call and calls back.
You can find more information on voicemail drops by doing a quick Google search or looking at the other voicemail drop services we recommend (they have info on their sites).
The other voicemail drop services we recommend can be found on the Battle of Fort Lauderdale training page (Bottom of Part 8, there is a direct link).