Okay allot of you probably already know how Paris Hilton's voicemail got hacked, if you don't this is how it was accomplished. :) It is a simple hack that involves spoofing the phone number you are calling from. If your cell phone voice mail system does not ask you for a password before logging you in to the account, anyone using Covertcall or a similar spoofing service, can spoof the caller ID number by calling the same number. The voice mail system thinks it is you calling your own number and automatically logs the attacker in to your voice mail box.. So word to the wise, turn off this feature on your phone to be safe.
CallerId spoofing can also be fun for playing practical jokes on people, or getting back at telemarketers who annoy the crap out of you ;)
Here are a couple of callerId spoofing sites to try: