Caller ID Spoofing

Caller ID spoofing is the practice of causing the telephone network to display a number on the recipient's Caller IDdisplay that is not that of the actual originating station. The term is commonly used to describe situations in which the motivation is considered malicious by the speaker or writer. Just as e-mail spoofing can make it appear that a message came from any e-mail address the sender chooses, Caller ID spoofing can make a call appear to have come from any phone number the caller wishes. Because of the high trust people tend to have in the Caller ID system, spoofing can call the system's value into question.

Learn more  http://www.fcc.gov/guides/caller-id-and-spoofing