One thing you can do is to go into the phone app and hit the little "i" inside the circle next to a number that's called you, scroll the whole way to the bottom and choose "Block this Caller." Then your phone won't ring or show a notification if that person tries to call you, if they leave a voicemail it will go to a "blocked messages' section in voicemail and you won't get any notification for this either.
To change the number, I found this page on Verizon's site:
From this site:
Request a new mobile phone number or transfer a number from your old carrier.