It comes with a SIM card. Yes Iphone 12 comes with the Sim Card included you don’t have to buy anything my phone came with the Sim Card and at best buy they helped me install it they set it up and transferred everything from my old phone to the new phone when I left the store phone was ready to use.
You can change your voicemail password by calling (800)-331-0500 from your iPhone. You’ll reach AT&T’s automated customer service menu which will ask for your phone number and billing zip code. A few seconds later, the message “Password Incorrect – Enter Voicemail Password” will appear on the display of your iPhone. Enter the last seven digits of your cellphone number to change your voicemail password. I strongly recommend that you read AT&T’s support article on changing your voicemail password before attempting to do so.

For the users to be able to do this from the phone or by calling into the system the users will have to have pre-recorded greeting for different status and switch between them from the phone or by calling in to the system. They can do that from the web client, windows client or an android client.
1 How do I reset my voicemail on Android?2 How do you set up voicemail on an Android phone?3 How do I change my voicemail message on my Samsung?4 How do I delete my voicemail greeting on Android?5 How do I delete voicemail on Samsung?6 How do I turn off voicemail on Samsung?7 How do I disable my voicemail?8 How do I switch off voicemail?9 How do I block voicemail on Android?10 How do I switch on voicemail?11 Why is my voicemail not working?12 How do I find out what my voicemail password is?