Voicemail drops are set up in Sales Sling as Campaign Dispositions.  To drop a prerecorded voicemail an agent will have to wait for the voicemail beep and then click on the voicemail drop disposition.  This will play the prerecorded message and disposition the call as such.  

1.  Find the campaign you wish to configure voicemail drop for in your manager dashboard and click on on the Actions drop down menu.

2.  Navigate to Edit Configurations>Dispositions.  Click +Add New Disposition and follow the configuration settings in the example below.

3.  When you are ready to record in a voicemail, click Start Record.  When you have a recording you are happy with click Save.

Note:  Be sure to not have any dead space at the beginning of your recording.  This translates to a delay in your voicemail, and will decrease the chance of people listening to it.  

Using VM Drop Disposition

1.  Press this disposition AS you hear the beep on your contact's answering machine.  Your recorded message begins playing about 1 second after you press the disposition.

2.  You will not hear the recording on your end, but the voicemail will still detect any Mic. noise you make.  So make sure you are not speaking over your recording, and disposition as normal.  

Your recording will finish out even if you disposition to the next call.