Someone I know has an account with one of these firms that offer cheap international calls by prepending a prefix to the number you dial. Frustrated that he couldn't register his work number with them to use it, he tried changing the caller ID his work phone issued to make it appear as if it came from his mobile, and lo, it worked - he could make international calls on his account. (His workplace is something telcoms related, and is set up to be able to change outgoing caller ID for legitimate work reasons)
Which, of course means that he could just as easily make international calls billed to anyone else's account, if he knows which phone numbers they have registered.
So, all that remains is to set up a premium rate phone service abroad, and let the scamming begin.
Please note, this scheme may not be legal.