You can authenticate your app with a super admin user, read your account extensions to detect the extension id of each extension you want to get a voicemail notification. Then subscribe for the voicemail event notification as shown below:

var eventFilters = [

      '/restapi/v1.0/account/~/extension/{extensionId_1}/voicemail', // e.g. 101

      '/restapi/v1.0/account/~/extension/{extensionId_2}/voicemail', // e.g. 102

      '/restapi/v1.0/account/~/extension/{extensionId_n}/voicemail', // e.g. n

]

ok. can you tell what are the steps for app authentication?