If a caller drops from an inbound call while navigating the IVR, the disconnect reason will show as abandoned_in_ivr.
Occasionally however, you will notice the disconnect reason as abandoned_in_ivr, but also notice that the caller was able to leave a voicemail message. If an inbound call never passes through a Ring To widget, the call will never "leave" the IVR. This is especially true when the IVR contains a TimeRule widget with an At Any Other Time leg. Calls that are routed to this leg, due to business hour settings or holiday schedule rules, can hit a Voicemail widget without ever passing through a Ring To Widget.
This scenario will result in a disconnect reason as abandoned_in_ivr even though the caller was able to leave a voicemail.