Not the OP, but if you are a doctor and need to ring a relative or a friend to inform them about a very sick patient, leaving a message without the opportunity for at least brief interaction isn't something particularly pleasant for either party. ( You either leave too nonspecific information that not being sure what to do, or too much that potentially is breaching patient confidentiality or is going to panic the recipient)
Ah that makes sense, thanks. I wasn't sure if there was something I didn't know about blocking unknown calls on iOS or Android that also prevented voicemails