If you have the numbers blocked in the system, they are not able to leave a voice mail. The block in place will intercept that call and the caller hears a recording that the person they called has a block. The only way a person whose number is blocked from your son's line would be able to leave a voice mail for him, is if they blocked or restricted the number they were calling from, or used a different phone to call him. Callers are also able to manipulate the number that appears on an incoming call. This is called spoofing and can be done using a computer or a third party application.
Because the call was not answered and went to voice mail, it will not show up in the call details for your son's line. This is due to the fact that when a call comes in and the user does not answer, no minutes are used. The only calls that appear on the bill are ones that connected.
You are able to check your son's missed call list to see if you can locate the number that called, or the fact that the number ws restricted by the caller. Please note that if the incoming call was made while your son's phone was powered off, it will not show as a missed call. The call will not appear in the call details on the bill or online.
Verizon Wireless offers a feature called Usage Controls that is $4.99 per line. It allows you to block restricted calls as well. You can view additional details about Usage Controls and all it can do by visiting:
I hope this information is helpful and addresses your concerns about your son's line.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.