You may find the listed duration for a voicemail doesn’t match the duration of the voicemail recording itself.
Our telecom provider processes the raw voicemail messages for us and filters out all the phone signaling and silence in the messages. This is a very conservative filtering, consisting only of silence and dial tones, so we are assured that no real content is ever lost. As a result, some voicemails show longer durations than the recording itself. Occasionally, this may even result in a voicemail file with a listed duration but no content.