Unknown ‘blocked’ caller Lewd Voicemail Messages

My daughter has been receiving very lewd sexual voice messages from an anonymous caller. He calls in the middle of the night and leaves these messages on her Voicemail. She has never answered the calls. The call comes in as ‘unknown’ with no number or info. It appears to be a blocked caller ID. Is there any way of revealing his number or blocking his number specifically. My daughter uses this phone for work and doesn’t want to block all unknown numbers. We have reported this to the police but she is still receiving these disturbing calls. Is there an app that would reveal his number or block him? I’ve contacted Verizon but they didn’t give many viable options. Help?