Don't worry I know exactly how you feel. My daughters ex has been making nuisance calls to me every ten minutes with the most vile language and name calling (he starts at 10pm and goes on and on for hours) - so I know how angry you feel. I had my number changed and bought a new mobile - so far so good............................he is a right terror though!
He has threatened our lives and the lives of our pets and horses - a right real ****** he is...but just that full of hot air and nothing else - I in turn told him he is full of hot air and I told him to take his 2 inch willy and put it away when he had finished playing with it - he was not amused to say the least, but since I have changed the numbers I have not heard from him.
The feeling and desire to get revenge can get to a point where you just want to boil over - but really it is not worth it...change numbers and move on.
Your mom must make a list of all calls for a period of 7 days dates, times, length of call and detail of calls - she can then give it to the police and they will act on it accordingly - with a mobile they will contact the mobile network and make it so his numbers cannot contact your mobile and as for landlines - hmmm not sure what they do... but they can do something.
revenge though it dreams as being soooo sweet is actually usually quite empty and unfulfilling...........................hmmm but worth a try?