If anyone ever harasses on the phone and their number says “withheld”, report the call and the exact time it was made to your phone service provider (or the police too if needed) and they can take action for you. The number is only withheld from your handset, the phone people will know exactly what it was if you give them the right time. They can't tell you who made the call and what the number is (because they're not allowed normally), but the will know the number (whatever it is), so don't worry. ^_^

Harassing people, stalkers, pranksters; whoever, they're not as anonymous as they think they are.

heh… Wrong numbers and calls to the wrong people… Years ago when friend's of my sister called (I didn't like them) I told them she'd moved. When they asked her number, I looked up the personals section of my newspaper and gave them the number of an escort agency. I thought that was hilarious. lol!
Even funnier, I did it more than once!

Heh, I was young when I used to do this, so don't hate me for being culturally insensitive or racialist, but when people called who I didn't want to speak too I'd go:
“Harro?! Harro? This Chinee Laundree, who you want? Harro? Who there?!!! This prank? I Caw the Porice!”
and then hang up… Too many Marx Brothers films when I was little I'm afraid.