The reason why Caller ID can be forged is that the calling device announces its CallerID number. The easiest way to get into spoofing CallerID is by using a SI

The reason why Caller ID can be forged is that the calling device announces its CallerID number. The easiest way to get into spoofing CallerID is by using a SIP-driven POTS gateway (very common 6 years ago, telecom moves very slowly, so I suspect they are still easy to find today). Some providers will validate the number you want to announce, and prevent you from announcing a number that they don't think you should be using. Others don't care at all. There are a lot of different telecom service providers, so finding one with lax policies isn't hard. Almost anyone who is actually operating a phone card business has all the equipment needed to do this, for example. Phone cards are incredibly competitive, so if you head over to your local telecom hotel you won't have much trouble finding people willing to take your money in exchange for steady business. The next time you are in a data center, take some time to talk to the people messing with the funky gear that doesn't look like any firewall or router you've ever seen.