According to the NY Times, Citibank was duped out of $27 million by an "elaborate" scheme that involved giving them his friends' phone numbers and having them pretend to be bank officials:
Swindles in which someone overseas seeks access to a person’s bank account are so well known that most potential victims can spot them in seconds.
But one man found success by tweaking the formula, prosecutors say: Rather than trying to dupe an account holder into giving up information, he duped the bank. And instead of swindling a person, he tried to rob a country — of $27 million.To carry out the elaborate scheme, prosecutors in New York said on Friday, the man, identified as Paul Gabriel Amos, 37, a Nigerian citizen who lived in Singapore, worked with others to create official-looking documents that instructed Citibank to wire the money in two dozen transactions to accounts that Mr. Amos and the others controlled around the world.
Fortunately for Citibank (and Ethiopia), the transfers were too large to be processed by the conspirators' banks and the money was sent back, so the scheme unraveled. In hindsight, they probably should have asked Citibank to send it Western Union.