Just like you can click on Phone Numbers --> Messaging --> Messaging Limits and see the maximum number of sms/mms messages you can send AND how many you have sent out of those limits, you should be able to do and see the same for voicemail drops
Furthermore, just like you can click on Phone Numbers --> Messaging --> Messaging Analytics and see a mini dashboard of (Sent, Delivered, Failed, Error, Received, Opt-out) for the sms/mms, you should be able to see the same for voicemail drops