An application and/or transmission left on a voicemail that causes problems with that smartphone. A targeted application via voicemail.
An unknown caller leaves a voice message and you retrieve it from your smartphone. There is no real message to understand, you think it was probably a wrong number. You instinctively delete it. However, did the message just install a VoiceMail Bomb? This could be a program, virus, or something else on the device.