One issue I have seen first hand is when an iPhone receives the voicemail notification, but not the voicemail itself. In an ideal world, the iPhone would tell you that the file isn’t available, but it doesn’t seem to do that in the world we live in.
8. Hello, you’ve reached [your name], [your job title] at [your company]. I’m sorry for missing your call, but if you leave a message that includes your name and contact information, I’ll get in touch as soon as I can.

You will be asked to delete your current Voicemail greeting. Click on the ‘Yes’ button that appears on the screen if you want to remove that. You can save your new recorded Voicemail greeting or can use the default recorded option. Wait and hear the new Voicemail greeting message that the instructions speak out. If you want to keep that recording as your Voicemail greeting, then save it. Or you can re-record the message to get a different greeting.
Calls will still technically be delivered to your phone, but your phone will not alert you to the call. This is a handset block and not a network block. You can put a network block on persistent telemarketers, but you need to contact your network provider for that. To view voicemails from blocked calls, go to Voicemail, then scroll all the way to the bottom and tap “Blocked Messages.”