Receive a message/file/voice call

Can I receive a message, file or a voice call if the sender is not in my contacts list?
No, you cannot. You can be contacted only by people that are in your contacts list. If someone is not in your contacts list and wants to communicate with you, you will be asked for a permission. If you grant a permission, this user will be added to your contacts list and you will be able to communicate with him/her.