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If you need to get voicemail notification for other extensions, the only way is subscribing for the /restapi/v1.0/account/~/telephony/sessions?direction=Inbound event notification. Then when you catch the event, parse it as shown in the example code below

...
// received thru your Webhook callback endpoint
var party = req.body.body.parties[
if (party.extensionId){
  if(party.status.code == "Disconnected"){
    if (party.status.reason == 'Voicemail'){
      // there is a voice mail
      ...
    }
  }
}