You might get a phone call from somebody claiming to be a technician with your telephone line provider. He will say they are running a test for a given reason and require you to cooperate, for a minute or so. The caller will ask you to press a couple of keys on the phone’s keypad. (eg. “9”, “0” and then “#”).
Depending on your state or country you are living in, the combination required just forwards your phone number to the scammers’ convenience. After you press the keys, they will be able to call any long-distance number on your account.
How to avoid: never do such a thing for somebody you don’t know and just make sure you get familiar with the prefixes used by your telephone line provider.